Add support for NETCDF3_64BIT_DATA in write_netcdf()

[xylar]

For performance, this involves first writing a NETCDF4 file and then using ncks to convert it to NETCDF3_64BIT_DATA (CDF5) format.

This merge also:

• adds a conversion of int64 format to int32, since the MPAS framework doesn't support int64
• adds documentation for write_netcdf()
• adds unit tests for write_netcdf(), including both CDF5 and int64 --> int32 capabilities
• adds nco as a dependency
• adds the logger to write_netcdf() calls in the mask creator (to keep output from conversion to CDF5 when it happens)

[xylar]

Testing

I tested this out by creating a new Icos30 base mesh in Polaris using this method. Both the mesh_triangles.nc and base_mesh.nc are identical before and after this change, but the file format it CDF5 as expected. See #632 (comment) for the script I used to test that all variables are identical.

[xylar]

Testing in Polaris

I was able to run the pr test suite on Chrysalis with these changes and by branch https://github.com/xylar/polaris/tree/write-cdf5, showing that the results were bit-for-bit identical to those with the Polaris main branch without these changes, and also that files were written in CDF5 format, as expected.

[xylar]

Further Testing

I have further tested these changes in a full workflow in Polaris that involved culling topography. This exposed issues with string variables in the python mask creator tools that I have now fixed in #638