This is a generic way of computing the average EIR for some subset(s) of the population strata. The object members is a membership matrix, like the residence matrix (see create_residence_matrix). If the residence matrix were passed to members, average_EIR returns the average EIR for the residents of each patch, which is could be different from what would be computed.
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.