TRT-571 - Update net2cog to process multiple variables.

[owenlittlejohns]

Github Issue: #32 (also one of the items mentioned in #35).

Description

This PR adds support for requests asking for multiple variables, either explicitly, or when a Harmony request specifies "all".

Overview of work done

There's a fair bit of shuffling around, but essentially the code changes are:

• If there is an empty list specified for variables, this is an "all" request, and so do all data variables.
• The _write_cogtiff function has been rescoped to just write a single COG. The iteration now takes place in netcdf_converter.
• I implemented the generate_output_filename from the harmony-service-lib-py module, which now has a reformatted suffix (note, this is a minor difference from converted). I removed a bunch of custom code that was geared around generating the output filename, as that is no longer needed.
• I consolidated test fixtures into a conftest.py.

Overview of verification done

• Updated unit tests (including consolidating test fixtures into tests/conftest.py). There are now tests that run to confirm multiple variables can be processed, either via explicitly requesting them or by using "all" in a Harmony request. These tests exist at the overall service level, and the netcdf_converter function level.
• I also have updated the net2cog regression tests (see: TRT-571 - Update regression tests for net2cog. nasa/harmony-regression-tests#131)

Overview of integration done

• Tested via the examples in the regression tests (see: TRT-571 - Update regression tests for net2cog. nasa/harmony-regression-tests#131).

PR checklist:

• Linted
• Updated unit tests
• Updated changelog
• Integration testing

See Pull Request Review Checklist for pointers on reviewing this pull request

[frankinspace]

Nice work, thank you for the contribution! Just a few comments to consider