add --rcluster-max option

The --rcluster-max option sets an upper limit on how many schemes the
relaxed clustering algorithm will analyse at any given step. The
default is to analyse all schemes.

This option has the desireable property of making the relaxed
clustering algorithm linear time w.r.t. the number of initial subsets.
E.g. rcluster with rcluster-max = 1000 will take at worst 1000N raxml
analyses (and usually a lot less).

Without this option (or if you don't specify it) the rcluster algorithm
is O(N-squared) which is a pain for very large datasets.

I haven't looked rigorously, but my suspicion is that setting
rcluster-max to some large value (like 1K or 10K) will make no
practical difference to the results. It just means you spend less time
searching for truly bad answers.