Fluctuating MRT: requires weighted-orthogonal method

This merge request makes sure that one gets an error message if an inappropriate LB method is chosen. Currently, create_mrt_orthogonal does not allow for explicitly choosing weighted vs. unweighted orthogonalization (see #5 (closed)), but I intend to fix that soon.

I still need to talk to Ulf Schiller to learn whether his specific orthogonalization of D3Q19 involved any secret tricks.

Closes #6 (closed).