Clean up and add method to spherical convergence analysis

[xylar]

This merge adds a polaris.mpas module that has:

• an area module that currently has a function area_for_field() for computing the area associated with a given data array from and MPAS output file. This is useful for getting different areas for fields on cells, edges and vertices.
• a time module where I have moved time_index_from_xtime(). I also added an optional start_xtime argument in case the start time isn't written to the output file. SphericalConvergenceAnalysis has been updated to use these.

This merge also adds a get_output_field() method to SphericalConvergenceAnalysis that can be overridden to compute fields based on MPAS output. This is useful, for example, in #120, where I need to compute the water column thickness (not an output variable from MPAS-Ocean) based on the ssh and bottomDepth variables.

Checklist

• [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 #120, which is based off of this branch. All tasks and steps including analysis run successfully with these changes:

QU:

Icosahedral:

[xylar]

The order of convergence is slightly better with area-weighting, which is interesting.

[xylar]

@cbegeman, no rush but I decided to break this out of #120 to keep thing cleaner. I think this should be quick to review by inspection.

[xylar]

Thank you, @cbegeman!