The matrix Q must be a sequential dense Mat, with all entries equal on
all processes (otherwise each process will compute a different update).
This function computes V(:,s:e-1) = V*Q(:,s:e-1), that is, given a set of
vectors V, columns from s to e-1 are overwritten with columns from s to
e-1 of the matrix-matrix product V*Q. Only columns s to e-1 of Q are
referenced.