mixvlmc 0.2.2

This is a bug fix release (see issue #79 for the main bugs):

• fix a bug in the C++ suffix tree implementation (subscript out of bounds in
  rare cases)
• avoid NaN production in loglikelihood() and related functions for the
  multinom predictive engine
• handle situations when a merged or local model has more degrees of freedom
  that the models it is supposed to replace (in covlmc)
• improve the interaction with the VGAM package in corner cases with
  degenerate models