Include the --thcovmat_if_present flag by default in vp-comparefits

[jekoorn]

Currently, even though MHOUs are included by default in the fit, the report does NOT use these to compute statistical estimators by default. One manually has to add the flag to vp-comparefits.
Setting it to True by default will be consistent to what is used in the fit.

[scarlehoff]

If you want flip the default and change it --dont-use-it-even-if-present
I don't have a strong opinion, although I favour the "less things by default" approach. The report is already quite heavy.