data interpolation functions

[aappling-usgs]

data interpolation is currently outside the scope of this package; users need to provide pre-interpolated data for the models to work. (see thread in #79)

1. it would be nice to help users with interpolation by providing functions to do it well. some functionality is already in mda.streams (e.g., combine_ts) and could be ported over

2. this is a problem that has been solved for many other cases already; there's probably a good existing package for it. look into existing options before getting fancy.