Fix the chi2 computation in the report when there is a thcovmat
Created by: andreab1997
At the moment the thcovmat is not considered for the computation of the chi2 in the report even if the flag use_thcovmat_if_present
is True in vp-comparefits
.
With this, the full thcovmat is used to compute the total chi2 and an appropriate cut thcovmat is used to compute the chi2 by process or by experiment (or even by dataset).