remove columns with too many NAs from bootstrap effectivemass fit

 effectivemass: During the bootstrapping of the effective mass fit NAs can appear. Although the fit routine offers the option of resampling any NAs with values from other bootstrap samples, if there are more NAs than non-NAs, this will fail. In this commit, columns (i.e. timeslices) are removed from the fits on the bootstrap samples if there is an insufficient number of values to resample.