Returns true if taxon `taxInd' is polymorphic for character `j', false otherwise.
Throws NxsException if `j' is too large (i.e. specifies a character beyond the last character for `uMatrix' row `taxInd'). Calls NxsUnalignedBlock::GetInternalRepresentation, so unless all you need is information about polymorphism, it is more efficient to simply call GetInternalRepresentation and extract the information you need from the returned vector. Note that `taxInd' should be the index of the taxon in the TAXA block. If data for that taxon has not been stored in this UNALIGNED block, then a NxsUnalignedBlock::NxsX_NoDataForTaxon exception will be thrown by GetInternalRepresentation.