Allow kmt variable name of "mask" and different size netcdf 2d reads

[anton-seaice]

PR checklist

• Short (1 sentence) summary of your PR:
  This is two bits of tidy up in the grid infrastructure, to make further extensions easier. I'm doing this PR now so the changes are easier to review.
• Developer(s):
  @anton-seaice
• Suggest PR reviewers from list in the column to the right.
  @apcraig
• Please copy the PR test results link or provide a summary of testing completed below.
  Still running, will update next week
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • Yes
  • No
• Does this PR update the Icepack submodule? If so, the Icepack submodule must point to a hash on Icepack's main branch.
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/. A test build of the technical docs will be performed as part of the PR testing.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please document the changes in detail, including why the changes are made. This will become part of the PR commit log.

This change allows for a single processor read of netcdf variables that are not of size nx_global by ny_global. This is to make it easier to extent the ice_grid routines.

This changes allows the mask variable in the kmt netcdf file to be called "mask" instead of "kmc". Both options now work.

This change splits reading the mask and grid into seperate subroutines, so the routine can be reused independently later.