Set string length in write_netcdf()

[xylar]

This pull request introduces enhancements and refactoring to the mpas_tools package, primarily focusing on improving the handling of string variables in NetCDF files, simplifying function signatures, and improving code readability. The most significant changes include adding support for configurable string length (nchar) in NetCDF files, removing redundant nchar arguments from various functions, and reformatting code for better clarity.

There is also a fix to a bug introduced in #638 related to adding non-string properties to mask datasets.

Enhancements to NetCDF handling:

• Added a new default variable default_nchar in mpas_tools/io.py to define the default string length for NetCDF files. Updated the write_netcdf function to include an optional nchar parameter for string length customization. [1] [2] [3]
• Modified the encoding logic in write_netcdf to handle string variables with the specified nchar and to improve compatibility with MPAS by converting int64 variables to int32.

Refactoring and simplification:

• Removed the nchar argument from multiple functions in mpas_tools/mesh/mask.py, such as _add_properties, _compute_region_masks, and _compute_transect_masks. The string length is now handled internally using default_nchar. [1] [2] [3]
• Updated function calls and definitions across mpas_tools/mesh/mask.py to align with the removal of nchar. This change simplifies the function signatures and reduces redundancy. [1] [2] [3]

Code readability improvements:

• Reformatted code in mpas_tools/ocean/moc.py for better readability, including breaking long lines, improving argument alignment, and using parentheses for multi-line expressions. [1] [2] [3]
• Updated the moc_southern_boundary_extractor function to use a more readable argument parser configuration.

These changes improve the usability, maintainability, and clarity of the mpas_tools package.

[xylar]

Testing

This has been tested with the MPAS-Analysis test suite. Without these fixes, the suite was failing during mask creation steps but with these fixes runs successfully.

[xylar]

Polaris' pr suite also runs fine with these changes.