Add sea-ice partition tools to conda package

[xylar]

This merge adds the following sea-ice partition tools to the mpas_tools conda package:

• prepare_seaice_partitions.py
• create_seaice_partitions.py
• fix_regrid_output.exe

It includes a small amount of clean-up of the python scripts to:

• add shebangs and make the scripts executable
• fix a typo
• remove deprecated string.atoi() call
• drop python 2.7 support in the form of print_function import (because new syntax is added that is not python 2.7 compatible)
• handle finding the fix_regrid_output.exe executable in the system path (e.g. from conda package)

Support for a CMake build of fix_regrid_output.exe has been added, as this is much easier to handle in the conda package than the existing Makefile.

The conda package has been updated to include gfortran and netcdf-fortran, needed to build fix_regrid_output.exe.

[xylar]

@akturner, I think this will make these tools a bit easier for E3SM Infrastructure folks to use for making sea-ice partition files. It is also a necessary first step in using these tools in compass.

Let me know if you have concerns about any of the changes to your scripts.

I will make a companion PR into compass that adds support for these tools there. I won't merge this PR until I have fully tested the other.

[xylar]

Needs considerably more work, I now see.

[xylar]

@akturner, please ignore this and focus on #484 for now.

[xylar]

@akturner, I rebased this onto master after merging #484. That might make these changes easier to follow, in case you're interested. Either way, I plan to merge once CI has completed.

Thanks for reviewing!