Add additional client tools for USGS NWIS services

This PR adds multiple additional NWIS services that return USGS metadata about sites (full name, drainage area, etc.), statistics (min, max, median, mean, etc.), streamflow peaks (day of peak and magnitudes), rating curves (stage vs. discharge curves), and National Weather Service Flood Stage categories (action, minor, flood, etc).

These additional data sources are very useful for conducting categorical evaluations using thresholds informed by statistics, peaks, or flood categories. Rating curves are required to translate flood stages to flood flows. Site metadata may be useful for any potential visualization tools, as well as being able to convert streamflow to areal averaged discharge (AKA "runoff" or "discharge per unit area").

@aaraney Let's discuss? Marking this as a draft, for now.

Additions

nwis_client.GenericClients module that includes interfaces for the Site, Stat, Peak, RatingCurve, and NWS Flood Stage services.

Removals

None

Changes

None

Testing

Basic testing using monkeypatched MockRestClient and MockResponse using local testing data.

Notes

Draft.

Todos

*subject to discussion

Checklist

[x] PR has an informative and human-readable title
[x] PR is well outlined and documented. See #12 for an example
[ ] 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)
[x] 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
[ ] Placeholder code is flagged / future todos are captured in comments
[x] Reviewers requested with the Reviewers tool :arrow_right:

NOAA-OWP / hydrotools