skip autocoalescence in recomb

In the SMC' algorithm, the whole point of the improvement with respect to SMC is the possibility of coalescing with the donor lineage.

Currently this is implemented "passively". It is allowed and, if it happens the recombination before anything happens to the donor lineage, the tree ends up absolutely identical.

It would be better to skip the whole event by taking into account its probability which is easy to compute (at least within the current eopoch). Like, do nothing unless we are sure we won't draw the same lineage. That should improve performance at least at little bit.