fixed numerical regression that has existed in some form since 14f6b67c3a3c4e17970e649df282409d3a81bc82
rephrased and DRYed the checks on the determinant and detrat for recalculating the inverse
increased ceperley_determinant_lower_cutoff, as this was causing occasional issues with numerical error

further investigation is needed to determine whether the appropriate
cutoff here depends on overall matrix norm or matrix size.
removed be_extra_careful, and always assume it is true so we never end up in a situation where
the matrix was thought not to be singular, but then is once the update is finished