fix nwis_client bug causing KeyError when station does not return data

[aaraney]

See #133 for the bug report. This PR resolves #133. Now, when a station does not have data, nwis_client returns a canonical dataframe with the correct data types to the user. Additionally, the user is warned that the request did not return data.

fixes #133

Additions

• canonical dataframe helper "private" method. This may be helpful for writing tests in the future/ asserting that dataframes comply with the standard. (see _create_empty_canonical_df)

Removals

Changes

• nwis_client.get returns a canonical dataframe with the correct data types in cases when no data is retrieved from nwis.

Testing

1. test_get_returns_empty_canonical_dataframe added. Uses mock to verify a canonical dataframe is returned.

Checklist

• PR has an informative and human-readable title
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• 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
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[jarq6c]

Looks good. Just one comment for discussion.

[jarq6c]

Merged with #135

[aaraney]

Awesome, thanks for merging these changes and redeploying!