Uniformize MPI interface (remove preprocessor #include)

[phil-blain]

PR checklist

• Short (1 sentence) summary of your PR:
  I changed the way MPI is included in the file ice_reprosum.F90 (both serial and mpi versions) from a preprocessor #include to a Fortran include statement.
• Developer(s):
  Philipppe Blain
• 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.
  I tested that the model compiles and runs correctly both in serial an mpi mode.
• How much do the PR code changes differ from the unmodified code?
  • bit for bit (expected)
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • 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 provide any additional information or relevant details below:

See #363 for some background. These changes are necessary to compile CICE using the NEMO build system.
I did not run the base suite because I don't think it's necessary; I think that the Travis suite will be sufficient. I can do the base suite it if the team thinks it is needed.