Tree kernel requires exact label matches

[ArtPoon]

In src/treestats.c, node labels (strings) are converted into integers using a deterministic heap sort and search. Thereafter, labels are compared as integer values within the kernel code. However, we may want match labels as patterns. For example, if a tip has multiple states A and B, it should match another tip in state A just as it matches tips in state B.

[ArtPoon]

I think the best way to implement this is to have a user-specified weight matrix. For example, in the situation where there are three labels A, B and C, we can use this matrix:

   A  B  C
A  1  1  0
B  1  1  0
C  0  0  1