allow forecast mean recruitment option for late recdevs

[Rick-Methot-NOAA]

Implementation of issue #471 caused late recdevs to be disallowed when using forecast recruitment option 4=mult*mean_over_yr_range
It is useful for some users to allow the forecast mean recruitment to be used for late recdevs, but this requires that that mean be calculated during the time series, not at the beginning of the forecast.

This requires creating code like:
if ( forecastmethod==4 && y==lastyearofrange) calculate that mean