Add minimal unit handling to nwm_client

[jarq6c]

This PR adds minimal unit handling to nwm_client. This is an incremental improvement that includes some housekeeping. Future enhancements will unify the gcp and http interfaces similar to nwm_client_new, and transition from google-cloud-storage to fsspec libraries.

@aaraney may be interested.

Additions

• unit_system parameter. Setting this parameter to "US" will convert dataframe values returned by get to cubic feet per second and update measurement_unit to "ft^3/s".
• UnitHandler class with methods to convert units.

Removals

• The old deprecated gcp_client has been removed.

Changes

• The gcp target for pip has been removed and google-cloud-storage is brought in automatically.

Testing

1. Value conversions are tested in test_units.py
2. pandas.Series conversions are tested in test_gcp.py

Notes

• Closes Investigate unit of measurement handling #137
• Link to Add method to transform units returned by gcp_client and nwis_client #113

Todos

• Unify gcp and http interfaces.

Checklist

• PR has an informative and human-readable title
• PR is well outlined and documented. See #12 for an example
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (see CONTRIBUTING.md)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output) using numpy docstring formatting
• Placeholder code is flagged / future todos are captured in comments
• Reviewers requested with the Reviewers tool ➡️

[jarq6c]

@hellkite500 sorry for the large number of files, I removed the gcp_client which is outdated. The main files with the new functionality are UnitHandler.py, gcp.py, and their associated test files.

[hellkite500]

Looks good as is. Just had some thoughts on the implementation of convert_values to ponder.