This is a generic way of computing the average PR
for some subset(s) of the population. The object members is a
membership matrix, like the residence matrix (see create_residence_matrix).
If the residence matrix were passed to members, average_PR_true returns
the average PR for each patch by residency.
The default, members=NULL, creates a \(1 \times\) nStrata matrix,
which takes the matrix over the whole population. If members is a
matrix with 1 column and nStrata rows, with only some elements set to one,
then it returns the average over those strata.