forecast_by_issue_time: Conduct site-level forecasting with the desired method
In kdayday/ppnwp: Post-Process Numerical Weather Prediction ensembles

For the given forecasting method, this function conducts the appropriate training method and forecasts for the test period. Exports .Rdata file of results to the "Runtime data" directory

1 2	forecast_by_issue_time(issue, ensemble, telemetry, sun_up, site, max_power, metadata, lm_formula)

`issue`	A lubridate time stamp of the issue time
`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`	An [issue x step] matrix of booleans
`site`	String, site name
`max_power`	Maximum load or site's AC power rating
`metadata`	A data.frame of forecast parameters
`lm_formula`	Formula for BMA linear regression