**Highlights**
* Improved interoperability with numpy:
  * `Field` now understands most of numpy's broadcasting
  * `Field` can be used as an argument to numpy's ufuncs.
* Import and export routines for `Field` incuding vtk, compatible with [paraview](https://www.paraview.org/).
* Coordinate mapping and transform for `Field`.
* Brand new `Multispecies.__call__` interface: This takes an expression, which is evaluated by `numexr`, increasing the speed of per-particle scalar computations strongly. It's also really user-friendly.

**Incompatible adjustments to previous version**
* `postpic.Field` method `exporttocsv` is removed. Use `export` instead.
* `postpic.Field` method `transform` is renamed to `map_coordinates`, matching the underlying scipy-function.
* `postpic.Field` method `mean` has now an interface matching `ndarray.mean`. This means that, if the `axis` argument is not given, it averages across all axes instead the last axis.
* `postpic.Field.map_coordinates` applies now the Jacobian determinant of the transformation, in order to preserve the definite integral.
In your code you will need to turn calls to `Field.transform` into calls to `Field.map_coordinates` and set the keyword argument `preserve_integral=False` to get the old behaviour.
* `postpic.MultiSpecies.createField` has now keyword arguments (`bins`, `shape`), which replace the corresponding entries from the `optargsh` dictionary. The use of the`optargsh` keyword argument has been deprecated.
* The functions `MultiSpecies.compress`, `MultiSpecies.filter`, `MultiSpecies.uncompress` and `ParticleHistory.skip` return a new object now. Before this release, they modified the current object. Assuming  `ms` is a `MultiSpecies` object, the corresponding adjustemens read:<br>
old: `ms.filter('gamma > 2')`<br>
new: `ms = ms.filter('gamma > 2')`
* `plotter_matplotlib` has a new default symmetric colormap

**Other improvements and new features**
* Overload of the `~` (invert) operator on `postpic.MultiSpecies`. If `ms` is a MultiSpecies object with filtered particles (created by the use of `compress` or `filter`), then `~ms` inverts the selection of particles.
* `postpic.Field` has methods `.loadfrom` and `.saveto`. These can be used to save a Field to a ` .npz` file for later use. Use `.loadfrom` to load a Field object from such a file. All attributes of the Field are restored.
* `postpic.Field` has methods `.export` and `.import`. These are used to export fields to and import fields from foreign file formats such as `.csv`, `.vtk`, `.png`, `.tif`, `.jpg`. It is not guaranteed to get all attributes back after `.export`ing and than `.import`ing a Field. Some formats are not available for both methods.
* `postpic` has a new function `time_profile_at_plane` that 'measures' the temporal profile of a pulse while passing through a plane
* `postpic` has a new function `unstagger_fields` that will take a set of staggered fields and returns the fields after removing the stagger
* `postpic` has a new function `export_vector_vtk` that takes up to three fields and exports them as a vector field in the `.vtk` format
* `postpic` has a new function `export_scalars_vtk` that takes up to four fields and exports them as multiple scalar fields on the same grid in the `.vtk` format
* `postpic.Field` works now with all numpy ufuncs, also with `ufunc.reduce`, `ufunc.outer`, `ufunc.accumulate` and `ufunc.at`
* `postpic.Field` now supports broadcasting like numpy arrays, for binary operators as well as binary ufunc operations
* `postpic.Field` has methods `.swapaxes`, `.transpose` and properties `.T` and `ndim` compatible to numpy.ndarray
* `postpic.Field` has methods `all`, `any`, `max`, `min`, `prod`, `sum`, `ptp`, `std`, `var`, `mean`, `clip` compatible to numpy.ndarray
* `postpic.Field` has a new method `map_axis_grid` for transforming the coordinates only along one axis which is simpler than `map_coordinates`, but also takes care of the Jacobian
* `postpic.Field` has a new method `autocutout` used to slice away close-to-zero regions from the borders
* `postpic.Field` has a new method `fft_autopad` used to pad a small number of grid points to each axis such that the dimensions of the Field are favourable to FFTW
* `postpic.Field` has a new method `adjust_stagger_to` to adjust the grid origin to match the grid origin of another field
* `postpic.Field` has a new method `phase` to get the unwrapped phase of the field
* `postpic.Field` has a new method `derivative` to calculate the derivative of a field
* `postpic.Field` has new methods `flip` and `rot90` similar to `np.flip()` and `np.rot90()`
* `postpic.Field.topolar` has new defaults for extent and shape
* `postpic.Field.integrate` now uses the simpson method by default
* `postpic.Field.integrate` now has a new 'fast' method that uses numexpr, suitable for large datasets
* New module `postpic.experimental` to contain experimental algorithms for your reference. These algorithms are not meant to be useable as-is, but may serve as recipes to write your own algorithms.
* k-space reconstruction from EPOCH dumps has greatly improved accuracy due to a new algorithm correctly incorporating the frequency response of the implicit linear interpolation performed by EPOCH's half-steps
* `plotter_matplotlib.plotField` allows to override `aspect` option to `imshow`
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