Issue in `CategoryCov.invert`

[luca-fiorito-11]

CategoryCov.invert fails for the following example:

sandy.get_endf6_file("jeff_33", "xs", 10010).get_errorr(err=1, mt=[102]).get_cov(multigroup=False)

with error:

Scipy.sparse RuntimeError: Factor is exactly singular

A similar issue can be found on stackoverflow at Scipy.sparse RuntimeError: Factor is exactly singular