Multistate constraints

[ms609]

@MaximizeParsimony.R currently has

  # Constraints are enforced as bipartitions, so only the two extreme states of
  # a character are read: taxa carrying an intermediate state are in neither
  # group and go unconstrained.  Say so rather than let the caller infer, from
  # `@param constraint`'s "compatible with each character", that a third state
  # groups its taxa too.
  if (nConsStates > 2L) {
    warning("`constraint` characters with more than two states are enforced ",
            "as the split between their first and last state only; taxa in ",
            "any intermediate state are left unconstrained.", call. = FALSE)
  }

Instead, treat such characters as additive, such that 000 111 222 decomposes to 000 000 111 AND 000 111 111.