Put low res topography options in a config file

[xylar]

This is much less confusing than having them hard-coded deep inside of global ocean.

Checklist

• Document (in a comment titled Testing in this PR) any testing that was used to verify the changes

[xylar]

Testing

I successfully ran the pr suite on Chrysalis with Intel and OpenMPI.

I verified that the config options in mesh.cfg for the Icos test case looked like they should (i.e. the low res topography data set, not the high res).