[BUG] clade probabilities with sampled ancestors

[hoehna]

Describe the bug
The MAP tree function doesn't compute the posterior probabilities of clades correctly when having sampled ancestors. Here we have a fake traces of 5 trees, all the same topology. The only difference is that one tip is set as sampled ancestor 3 out of 5 times. All PP of clades should be 1.0, but aren't.

Two more issues:

1. The first tree is omitted even though the burnin is 0.
2. The node age of the sampled ancestor is set to the non-sampled ancestor state. Why?

To Reproduce

trace = readTreeTrace("TryTreeWithSA.trees",burnin=0,treetype="clock")
mapTree(trace,file="map.tre")

TryTreeWithSA.trees.zip