Static method to get the low-level function that fits the PLM.
 Any subclass model must provide this method, which should return
 a function that accepts a numeric vector of length K, where K
 is the number of probes.
## Static method (use this):
## SingleArrayUnitModel$getFitUnitGroupFunction()## Don't use the below:
# S3 method for SingleArrayUnitModel
getFitUnitGroupFunction(...)
Returns a function.
Not used.
For more information see SingleArrayUnitModel.