Add RelPP layer for each PP group

[jeroens]

In Ecosim users can apply PP forcing to individual PP groups. Ecospace has only one RelPP layer; to better force the production of different Producers we need to have a RelPP layer for year producer group.

The Ecosim code must already resolve ambient nutrients; Ecospace can borrow the same principles

[jeroens]

Misunderstanding! The Ecospace RelPP layer has the same effect as the Nutrient Loading forcing function in Ecosim. To force the biomass of a specific producer group one should use a time series in Ecosim, and a spatial-temporal time series in Ecospace. No changes needed; one just needs to have the spatial temporal data framework