[Feature] Adjustments to SRM for decoction, boil

[DougEdey]

Converted from SourceForge issue 1632632, submitted by dlevey

It would be nice to be able to make adjustments to final SRM. Even assuming that initial reading are accurate based upon malt yield, it's possible to darken the final product due to concentration/carmelisation in the boil, or by decocting during the mash. The adjustment could have an impact upon style conformity.