Add a new utility to create ESMF and SCRIP files for MPAS-A

[MiCurry]

This merge adds a Fortran utility that can generate ESMF and SCRIP files from an MPAS mesh. Although there already exists a utility that can create SCRIP files from an MPAS mesh, that utility uses Conda, which is not part of the MPAS-Atmosphere's current workflow.

Currently, this tool is used to generating SCRIP files for MPAS in CESM.

[MiCurry]

@xylar Althought I don't think there is, let us know if you think there will be a conflict with this tool and the create_SCRIP_files utility.

[xylar]

@MiCurry, no, there is no relation between the two tools so there should also be no interference. Thanks for asking!

[mgduda]

@MiCurry Thanks for putting together a Makefile and PR for this. I have just a few minor suggestions:

1. Should we add a .gitignore file with mpas2esmf as its only line?
2. Should the Makefile set FC, e.g., based on the output of $(shell nc-config --fc) as we do for grid_rotate, or is the expectation that the user will set FC on the command-line?

[MiCurry]

@MiCurry Thanks for putting together a Makefile and PR for this. I have just a few minor suggestions:

1. Should we add a `.gitignore` file with `mpas2esmf` as its only line?

2. Should the `Makefile` set `FC`, e.g., based on the output of `$(shell nc-config --fc)` as we do for [grid_rotate](https://github.com/MPAS-Dev/MPAS-Tools/blob/master/mesh_tools/grid_rotate/Makefile), or is the expectation that the user will set `FC` on the command-line?

Those both sounds like good ideas! I wasn't aware of nc-config --fc. I'll add them both. Do you think its worthwhile to also have the same FFLAGS used as grid_rotate?

[mgduda]

If we can avoid having to add FFLAGS, I think that's preferable, as it makes the default Makefile compatible with multiple Fortran compilers. With the grid_rotate tool, the FFLAGS only work with the GNU compiler, and users need to manually change these when switching to other Fortran compilers.

[MiCurry]

@mgduda sounds good! I just pushed (force pushed) an update to this branch with the changes you requested. I also added a short README. Let me know if there is anything else.