Added two transmembrane domain specific substitution matrices: PHAT75/73 (dx.doi.org/10.1093/bioinformatics/16.9.760) and SLIM161 (dx.doi.org/10.1093/bioinformatics/17.suppl_1.S182)
Note that SLIM161 is a non-synonymous matrix.
Added the function "mirror_matrix", which fills in the reverse key-orders of a matrix dictionary.