fix nwis_client bug causing KeyError when station does not return data -- Updated

[jarq6c]

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

Addenda: Updates empty dataframe method to return a non-canonical dataframe if specified by the user. Necessary for #117

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
• 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 ➡️