train_constant_bma: Subfunction to get single, constant BMA model for all...
In kdayday/ppnwp: Post-Process Numerical Weather Prediction ensembles

Subfunction to get single, constant BMA model for all time-points

1 2	train_constant_bma(t_idx_series, ensemble, telemetry, sun_up, site, max_power, metadata, lm_formula)

`t_idx_series`	Series of time indices to forecast, relative to the telemetry time indices
`ensemble`	A list of data=[issue x step x member] array of all ensemble data (historical + test) and issuetime=vector of POSIXct time stamps
`telemetry`	A list of data=vector of telemetry and validtime=vector of POSIXct times
`sun_up`	A vector of booleans, indexed by telemetry valid times
`site`	String, site name
`max_power`	Site's AC power rating or maximum load
`metadata`	A data.frame of forecast parameters
`lm_formula`	Formula for BMA linear regression