matrix into a form usable by MLwiN for multiple membership models,
namely a data.frame with (a) columns containing membership IDs (if first row matrix is
0 1 1 0 1 1, then first row of generated ID vectors would be, say, 2, 3, 5, 6)
and (b) columns containing weights (in this example, if standardise = TRUE, then first
row of generated weight vectors would be, say, 0.25, 0.25, 0.25, 0.25, otherwise first
row of generated weight vectors would be, say, 1, 1, 1, 1).matrix2df(mat, standardise = FALSE, idstub = "id", weightstub = "weight")TRUE, ensures the row sums to one; defaults to FALSE.id.weight.df2matrix