Generate cohortData table per pixel:
A data.table with columns: pixelGroup, ecoregionGroup,
speciesCode, and optionally age, B, mortality, aNPPAct, and sumB.
A RasterLayer with pixel values equal to a pixel group
number that corresponds exactly to pixelGroup column in cohortData.
the columns in cohortData that define unique cohorts
A logical indicating whether some internal tests should be run to
ensure the function is running correctly.
Default: getOption("LandR.assertions", TRUE).
An expanded cohortData
data.table with a new pixelIndex column.