Add minvisratio feature
Created by: aroffringa
This allows skipping the solve for solution intervals with fewer unflagged visibilities. This fixes #83 (closed). So far I've done some quick testing with minvisratio=0.6, which seemed to work as usual, and minvisratio=1, which indeed flagged almost everything, so it seems to work on first order. Awaiting further validation by @revoltek.