Added member function to Phylotree to add custom haplotype descriptions.
Added member function to Phylotree to add custom haplotype descriptions.

Custom haplotypes can now be added to the Phylotree.hap_var table. We can't
add an entry to the tree. Any unspecified positions are assumed to be
reference.

svohr/mixemt,svohr/mixemt,svohr/mixemt