Nested-grid-plotter

------------------

* IMP: `add_xaxis_twin_as_date` is now deprecated and replaced by the more
flexible `ticklabels_to_datetime` and `add_twin_axis_as_datetime`
functions. The tutorials and tests have been updated.
* IMP: `make_x_axes_symmetric_zero_centered` and
`make_x_axes_symmetric_zero_centered` have now possibility to ensure minimum
symmetric axis limits through the `min_xlims` and `min_ylims` keywords respectively.

------------------

* FIX: bbox_extra_artists when using savefig method.

------------------

* ENH: Add a `get_axes` interface to `NestedGridPlotter` class.
* ENH: Provide a `add_letter_to_frames` utility function.
* FIX: RunTimeError Can not put single artist in more than one figure when using
* NestedGridPlotter `add_fig_legend` method and using the main figure.

------------------

* FIX: Prevent figures an subfigures legend cut-off by th figure box when saving images
to disk.

------------------

* FIX: Typo = symetry to symmetry in keywords.
* FIX: Colorbar scaling = now supports any norm and some duplication has been removed,
some warnigns added.

------------------

* FIX: Selection of data in animations - when the amount of data is
larger than the number of frames. The fix ensures that the first frame
is the first data element and that the last frame is the last data
element, all other frames matching evenly spaced data element in between.
* DOCS: update animated_plotters.ipynb notebook