Added an EdgeTopologyMove that simultaneously does a TopologyMove and a
change to the edge lengths of the two switched vertices. However, this
doesn't seem to help much, so it's disabled by default. Currently there
is also no automatic tuning for this move, since it is not obvious how
to do so -- quite possible something like this might work better if it
were optimised a bit more effectively.

Also added an AllEdgeMove, which is supposed to multiply all the edge
lengths by a constant multiple. However, there is currently a bug with
this (something to do with how the cached likelihoods are updated for
the whole tree), so it's disabled at the moment.