format_rec_df: Format reconciled dataframes
In camroach87/gefcom2017: Functions for GEFCOM 2017

This function is used after reconciliation to convert data to the correct format. Used by hts_reconciliation function.

1	format_rec_df(hts_raw, hts_rec, variable_cols, pred_col_name = "Prediction")

`hts_raw`	the raw time-series data.
`hts_rec`	the reconciled data frame.
`variable_cols`	variable column. Columns that provide a unique identifier for each time-series forecast. Inherited from hts_reconciliation().
`pred_col_name`	the name assigned to the prediction column. Defaults to "Prediction".