Include secondary weather data when fitting for PWV

[djperrefort]

pwv_kpno currently uses a linear model to primary PWV calues from secondary receivers, but does not consider the inclusion of any non-pwv data (e.g., surface pressure, RH, and temperature). We should explore how including this data can improve our predictions.

Action items:

• Perform a PCA to understand what parameters from our data have the biggest impact
• Try a few different fitting techniques / models (multivariate regression, SVR, etc.)
• If feeling ambitious, explore some additional machine learning options