For automated selection of descriptors - setting cutoff

[ddmorgan]

Set so that features stop at some "converged" value. Convergence should be set by whichever gives the the lowest of below

1. A max value (50 default)
2. RMS error between n+N and n descriptors changes by E < A x std dev. for N steps past n* (ie, n*,n*+1,...n*+N have changes < E) (Defaults of A=0.01, N=5).
3. RMS error between n+N and n descriptors changes by E < B * avg error in the mean of RMS error across CV. Or something like this? Not sure?