Resolve nwis service raising pandas FutureWarning

[aaraney]

pandas >= 1.5.0 was raising FutureWarning in nwis_client.get because of ambiguity in setting a column's values in-place or replacing the column's underlying array. Specifically, the warning is:

FutureWarning: In a future version, 
`df.iloc[:, i] = newvals` will attempt to set the values inplace instead of always setting a new array. To 
retain the old behavior, use either `df[df.columns[i]] = newvals` or, if columns are non-unique, 
`df.isetitem(i, newvals)` dfs.loc[:, "value"] = pd.to_numeric(dfs["value"], downcast="float")

This PR fixes that issue.

fixes #209

nwis_client

Changes

• pandas >= 1.5.0 no longer raised FutureWarning in nwis_client.get. see #209.

Testing

1. test added to verify warning is not raised in nwis_client.get.

Checklist

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

[aaraney]

So, the one of the nwm_client_new's tests are failing.

FAILED python/nwm_client_new/tests/test_NWMFileProcessor.py::test_get_dataset - AttributeError: 'EntryPoints' object has no attribute 'get'

I am going to move this to an new issue, so we can track it there. The "good news" however, is that the nwis_client tests passed.