New option to TrySplits when nChanged hits a certain threshold

[thesamovar]

Currently, TrySplits is tried when nChanged gets to 0, but actually it would be sensible to have a higher threshold. This could be a parameter.

Typically, TrySplits will lead to much larger nChanged, so this would potentially save quite a lot of computation.

[thesamovar]

Note that we don't want to repeatedly TrySplits, so we have to keep track of the previous value of nChanged and only TrySplits if nChanged<=threshold<oldnChanged.

[nsteinme]

Could the right approach be to check whether TrySplits did actually come up with any new clusters - and if not, to ignore the threshold and let nChanged go to zero (before TrySplits a final time)?

Generally I think this is a very good enhancement idea!!