Calculate site shade

calcSiteShade(currentTime, cohortData, speciesEcoregion, minRelativeB)

Arguments

currentTime	The current simulation time e.g., `time(sim)`.
cohortData	A `data.table` with columns: `pixelGroup`, `ecoregionGroup`, `speciesCode`, and optionally `age`, `B`, `mortality`, `aNPPAct`, and `sumB`.
speciesEcoregion	A `data.table` with `speciesEcoregion` values.
minRelativeB	a `data.frame` with the cut points to classify stand shadiness.

cohortData table with a siteShade column