add support for parallel in find.clusters

[timflutre]

I did the same as in glPca (that is, I used parallel::mclapply and added the arguments parallel and n.cores). I also updated the documentation and DESCRIPTION.

[zkamvar]

Hello! Thank you for this PR. I think this is a good idea; would you mind if I made some modifications before I merge this?

Specifically, there is now duplicated code for calculating the WSS that I would like to turn into a stand-alone internal function.

[timflutre]

Hello, thanks for your answer! Yes you can modify it: turning WSS into a stand-alone function is definitely a good idea!