Move mutations on a single recombinant match above parents

[jeromekelleher]

Currently mutations get pushed below the recombination node, which is probably less parsimonious that putting them on the branches back to the parents.

[jeromekelleher]

This is probably not a good idea in the short term as mutations that occur on these partial edges won't be available for the reversion push and overlap hueristics. It is definitely something to remember though.

[jeromekelleher]

See #86 e.g.