Finish & connect iterating solver

This MR fixes a few bugs in the iterating solver, changes it to use single precision and adds parset options to select the iterating solver. I have tested this on a small (Rapthor) data set, and the results look good (once all the bugs were fixed). The unit test also has a stricter tolerance now -- the reason the tolerance was increased was in fact because of the bugs, not because the algorithm is intrinsically inaccurate.