Move hardcoded last forecast day to configuration

[bartnijssen]

https://github.com/bartnijssen/uw-hydro-forecast/blob/master/tools/bin/process_flux_and_rout_output.pl

## Usage <CURRFCST> <LASTFCST> <BAS> <COMMON_DIR> <XYZZDIR> <XYZZFILE> <PLOTDIR> <PLOT_SCRIPT>
### Assuming that last forecast was made a week before the current one
### Should not be hardcoded!!
($L_fcst_yr, $L_fcst_mon, $L_fcst_day) = Add_Delta_Days($Cyr, $Cmon, $Cday, -7); 
$LAST_FCST_DATE = sprintf("%04d%02d%02d",$L_fcst_yr, $L_fcst_mon, $L_fcst_day);