Returns true if the state at taxon `taxInd', character `j' is the missing state, 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 missing data, it is more efficient to simply call GetInternalRepresentation and see if the returned vector is empty. 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.