Dynamically detect last available data from USGS sensor data feeds

[fungjj92]

Sometimes, USGS sensors fail to collect up-to-date data.
In order to get any real, past sensor data, the API requests for data must be given a start date query param that includes date(s) for which there is real data. In #63 we added date constants since when is it known that all the relevant sensors have real data. This is not sustainable. The chosen dates will always work, but become outdated over time insofar that we'll poll for more data than we need. I did not find a programatic and dynamic way to discover these dates.

Figure out a better way or alternative to the current/initial set up.

Connects #63