Horizontal colorbar support for all relevant plot functions with colorbar

[lassejsc]

Added option for horizontal colorbar for all the plot functions with cbaxes kword, changes based on #237.
Currently changed:

• plot_vdf
• plot_colormap
• plot_colormap3d(cherry picked from Horizontal colorbar support #237 for one of the scripts in scripts folder and added to the main package also)
• plot_neutral_sheet
• plot_ionosphere

Also adds colorbar calls to testpackage.

Example plot_vdf when cbaxes and axes are passed to it

Also added semi sensible default placements for horizontal colorbar if only cb_horizontal=True is passed to these. Note that for vdf and vdfdiff this may not always look good as I believe the figure size comes from the data.

vdf:

vdfdiff:

Here I reduced the number of ticks when plotting with horizontal bar, because matplotlib is giga annoying to work with and I could not easily a) make the ticks have decimal precision of just 1 b) widen the colorbar. This may only be an issue if the main plot is too narrow and it depends on the data I presume.

colormap:

colormap3dslice:

plot_neutral_sheet:

ionosphere:

Note that this is after the change in #363 , which is not included in this PR

[alhom]

Please add example figure outputs for reference.

[alhom]

And please add testpackage lines where available (plot_colormap, _vdf)

[alhom]

Very good! Two more things:

1. For reference and for exercise, also please add example outputs for the other plot types you have modified.
2. Please revert plot_colormap3dslice.py to master and cherry-pick the original edit in its place to conserve author/blame history.