Add additional USGS webservices

[mroberge]

The USGS provides several additional services, including:

• Flood stage: returns a dictionary of stage heights for action level, flooding, moderate flooding, and severe flooding for about 5,000 gauge sites.
• Real-time streamflow: current discharge, expressed in cfs and as a % of mean & median.
• Current flood and highflow: similar to real-time, but returns only flooding sites
• Average streamflow for 7, 14, and 28 day periods: similar to real-time, but returns either 7, or 14, or 28-day average discharge
• Flow change: change expressed as a % change.

These services are described here: https://waterwatch.usgs.gov/webservices/
The most useful service seems to be flood stage. This could be used to annotate flow duration charts or other figures.

[mroberge]

flood stage was covered by #87