Change Tree.mrca signature to take > 2 args

[jeromekelleher]

Following the discussion in #1339, we should change the signature of Tree.mrca to accept two or more nodes. The initial implementation can be something like

def mrca(self, *args):
   if len(args) < 2:
        raise ValueError("Must supply at least two arguments")
   return functools.reduce(self._ll_tree.mrca, args)

We're not going to break anyone's code by doing this, and it's an obvious generalisation. We can make a more performant implementation in the future if we like, but this'll do fine for now.

[hyanwong]

This would be useful to find the MRCA of all samples in the tree (which could be different from the root if there are unary nodes above the grand MRCA). Would it be much slower than looking at the root and descending until we find a non-unary node, though?

[jeromekelleher]

I think we should make this a different function (and issue).