In linguistics, we tend to measure times in years, so heights are in the
order of magnitude of 100s to 1000s, not the 0.1My often found in
biology. In order to avoid overflow errors, the log should be taken
ASAP, and in fact this does not make the computation more expensive.

This should fix one of the symptoms of CompEvol/beast2#984 and also CompEvol#16

The SAWilsonBalding operator was permitted to turn sampled ancestors
into normal tips. For an analysis where certain tips should be kept
sampled ancestors, add a Boolean input (default: false) that aborts the
operator proposal when it is about to do this change.