Enhance spherical tracer transport test viz

[cbegeman]

Add options for over/under colors for the colormap used in spherical mpas visualization. These were added to be able to more conveniently visualize over and undershoots in the tracer advection scheme tested in the sphere_transport tests.

• Testing comment in the PR documents testing used to verify the changes

[cbegeman]

Testing

The sphere transport viz step was successfully run and shows the over/under colors for the tracer field but not for the layer thickness field as expected.

[xylar]

This looks great to me by visual inspection. My only suggestion would be to add it to the developer's guide if there's an appropriate place.

[xylar]

Maybe add something to the developer's guide here:

polaris/docs/developers_guide/framework/visualization.md

Lines 133 to 155 in a010381

	```cfg
	# options for visualization for the cosine bell convergence test case
	[cosine_bell_viz]
	# colormap options
	# colormap
	colormap_name = viridis
	# the type of norm used in the colormap
	norm_type = linear
	# colorbar limits
	colorbar_limits = 0.0, 1.0
	```
	`colormap_name` can be any available matplotlib colormap. For ocean test
	cases, we recommend importing [cmocean](https://matplotlib.org/cmocean/) so
	the standard ocean colormaps are available.
	The `norm_type` is one of `linear` (a linear colormap) or `log` (a logarithmic
	colormap).
	The `colorbar_limits` are the lower and upper bound of the colorbar range.

And then ideally update the config options in the 4 sphere_transport test cases, e.g.:

polaris/docs/users_guide/ocean/tasks/rotation_2d.md

Lines 226 to 236 in a010381

	[sphere_transport_viz_tracer]
	# colormap options
	# colormap
	colormap_name = viridis
	# the type of norm used in the colormap
	norm_type = linear
	# colorbar limits
	colorbar_limits = 0., 1.

[cbegeman]

@xylar Thanks for reviewing! Docs have now been updated.