Split bathymetry subroutines into seperate file

[anton-seaice]

As a precursor to #807, we should split ice_grid into several files, as its >4000 lines.

This PR is for comment on the approach to do this with least interruptions, as several groups have their own copies or changes in their CICE forks. If this change looks ok, I will do the same for other sections of the ice_grid file (e.g. make an ice_gridbox.F90, ice_grid_average.F90 etc)

The changes won't change the interface to the ice_grid module, nor change the functionality of any routines.

PR checklist

• Short (1 sentence) summary of your PR:
  Move bathymetry routines into ice_grid_bathy.F90 file
• Developer(s):
  @anton-seaice
• Suggest PR reviewers from list in the column to the right.
  @apcraig @phil-blain @daveh150
• Please copy the PR test results link or provide a summary of testing completed below.
  None yet
• 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.
  Move bathymetry routines into seperate ice_grid_bathy.F90 file to improve readability and maintainability