post-processing features / cleanup

From the release notes:

• meshseries.plot_time_slice has a new user interface - see the update example or the API reference. The interpolate argument was removed, as it tempts you to be used as a default (as it produces a nice image), but in doing so, can easily lead to wrong conclusions as the interpolated image might differ significantly from the raw data.
• generalized meshseries.animate: its now a free standing function (plot.animate) and can take any plotting function - see the updated example or the API reference
• Variables now have methods to create Variables related to absolute error, relative error and analytical solution corresponding to the parent Variable
• Variable.transform now also works on MeshSeries
• plot.line now can also handle MeshSeries - this allows to draw an array of lines in one call (e.g. lines for each pt in the MeshSeries over time or lines for each timestep over a spatial coordinate)
• plot.contourf now only returns a figure if it wasn't given one as an argument
• improved axis labelling for shared axes (only the outer axes should get labelled now)
• new: meshseries.resample to interpolate the meshseries to new timevalues
• new: extract_probe to create a new meshseries consisting only of points and probed data at these points

1. Feature description was added to feature list in future release notes
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented as a jupyter notebook?
4. API-breaking changes documented in future release notes