handle rating curves

[gdelaplante]

AquaCache currently cannot hold rating curve information. This metadata is sometimes useful for researchers and industry, so tables could be created to hold this information. Functions and workflows also need to be created to periodically check and synchronize rating curves from the WSC, Aquarius, or other sources.

A (very) draft table structure exists in file dev/rating_curves.R. Missing is notably a means of identifying the source of the rating curve, how to fetch/compare new curves on remote data stores, and updating of rating curves.

Function YGwater::aq_download_curves partly does this for Aquarius rating curves, though it is limited in how it handles equations.