Added 'robust' re-fit of coefficients using RIRLS

Not sure if this will work in practice, but maybe it will fix some of the
issues caused by missed clouds/shadows. If we miss cloud/shadow, and we
don't false positive call it a change, then our regression result will be
wrong (too high, usually) and our RMSE will be too large.

This might kill fits for normally noisy environments (ag, wetlands), but
maybe it works better for forests
Fix for 'robust' records if all coef are 0
Staging for some omission/comission tests
Typo fix
Moved ggplot import from toplevel to plot functions
Fixed default test indices calculation

YATSM doesn't take in Fmask (needs to be pre-screened) so no need to ignore
last band
Removed useless variable