Reorder attr checking to avoid restart file issues

[andrewdnolan]

Only need to check for is_periodic with non-spherical meshes. Reordering will avoid issues with restart files.

Fixes #31.

[andrewdnolan]

Testing

The code snippet:

import mosaic
import xarray as xr
mesh_file = '/global/cfs/cdirs/e3sm/inputdata/ocn/mpas-o/ARRM10to60E2r1/mpaso.ARRM10to60E2r1.rstFrom1monthG-chrys.220802.nc'

ds = xr.open_dataset(mesh_file)
descriptor = mosaic.Descriptor(ds)

Now works without any issues. @xylar Checking a restart file from the periodic simulation could be good, since that's an edge case not covered here. Though that seems like a really rare thing in practice.

[xylar]

We definitely do restart tests on meshes that are periodic so I think maybe it's just that the periodic attributes aren't written out for spherical meshes (where they are nonsense). But I haven't been able to prove that.

[xylar]

This change makes a lot of sense to me.