Add time-varying minimum output to HYDRO

[GenXProject]

Add a modifier to implement the minimum output by multiplying with the capacity factor.

[cfe316]

One way to do this would be to change Generators_variability.csv into a more general Resources_timeseries.csv. (Or keep the same name...)

Right now Generators_variability gets turned into a matrix which is precisely indexed by the resource index y. I don't think there's any good reason for the data structure here to be a matrix; instead it should be a dictionary of 1D time series arrays (or perhaps a dataframe). Values can be looked up by dictionary key (such as UtilitySolar_1_ISONE or Hydro_2_WECC:MinPower ) and time step.