allow for different update styles

[willwerscheid]

Currently the order of updates in backfits are l1, f1, l2, f2, ..., lk, fk. explore using:

1. l1, l2, ..., lk, f1, f2, ..., fk
2. (l1, l2, ..., lk) n times or until some convergence criterion is reached, (f1, f2, ..., fk) possibly also multiple times

(2) should give us some speed-up since we can re-use the computation of YF and Y'L over iterations. See Gillis's book, Algo 8.2 and text for example.