Add global map plots

[xylar]

This merge adds a new MappingFileStep for making mapping files between all the grid types supported by pyremap.

It also adds a function, plot_global, for plotting global arrays on a lon-lat grid.

Finally, this merge adds VizMap and Viz steps to the cosine bell test case to demonstrate the new capabilities. The result are images (with land plotted overtop, though that could easily be removed with a flag to the plotting function) from both the beginning and end of each simulation.

Checklist

• [x] User's Guide has been updated
• [x] Developer's Guide has been updated
• [x] API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• [x] Documentation has been built locally and changes look as expected
• [x] Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I ran a reduced-resolution version of the cosine bell QU test on my laptop, resulting in plots like the following:

QU210 init:

QU210 final:

QU300 final:

[xylar]

@cbegeman, thanks for the review. I started writing documentation for this and felt pretty dissatisfied with the way that remapping is handled (specifically the clunky way that I had to make up a name for the mapping file before knowing things like the output grid resolution). If time permits before the hackathon, I'm going to do some other work first, then come back to this. I'll make sure your review is incorporated.

[xylar]

Re-testing

This has been tested in conjunction with #39 and #40. The new VizMap and Viz steps run as expected on a sequence of resolutions on my laptop (210, 240, 270 and 300 km).

[xylar]

@cbegeman, okay, just letting you know that I'm going ahead with this one as soon as CI passes.