Add um2netcdf entry point(s)

[truth-quark]

For simplicity, this issue is for um2netcdf entry points, separate to the ESM1.5 conversion driver entry points.

Tasks for adding entry points for um2netcdf are:

• Move cmd line parsing & process() call into a new main() function
• Update conda (?) config to allow direct calls to um2netcdf (via console-scripts)
• Add CI test to verify simple command(s) can be called from the env e.g. un2netcdf --help

Other general thoughts:

• Moving the parsing & process() call to functions provides an interface for testing
  • As of mid Aug 2024, cmd line parsing cannot be tested as it's in __main__
• Adding main() provides a top level function
  • this facilitates splitting process() into workflow sub functions under main()
  • helps simplify testing with reduced need for complex test setups & mocking
  • new workflows can be composed from the components (do future ESM versions need custom um2nc workflows?)

[truth-quark]

Resolved by #86.

[truth-quark]

This relates to #81 too.