Add mapping file step

[xylar]

This merge adds the MappingFileStep to the framework. It can be used to create mapping files between various grids and meshes.

The merge also adds an example of its usage to the cosine bell test case. Each MPAS mesh is remapped to a regular lon-lat grid with 0.5 degree resolution (by default -- a config option can be used to change this) using conservative remapping (again, controlled by a config option).

This merge builds off of dependencies, added in #39

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

This has been tested in conjunction with #39 and #34. 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, I pulled this out of #34. I hope there aren't any surprises here. If you wouldn't mind giving it a quick glance when you can, I'd appreciate it. Then, I'll merge and rebase #34 so it can go in cleanly. No need to re-review that one.

[xylar]

Thanks for taking a look @cbegeman!