Test new max_nlev construct in Bryan-Lewis test

Trying to run Bryan-Lewis with max_nlev != nlev in the columns uncovered a
couple of issues:
1) max_nlev wasn't a recognized attribute name in cvmix_put (actually in
cvmix_utils::cvmix_att_name)
2) cvmix_io needs to be hard-coded to write 1:nlev or 1:nlev+1 instead of full
arrays.