covariance_matrix_plot runcard fails when executing procs_chi2_table

Created by: andreab1997

Running the covariance_matrix_plot runcard fails when entering in the function groups_chi2_table https://github.com/NNPDF/nnpdf/blob/9840fc69dfcbcf25ebb30236916c4f1fc65a1785/validphys2/src/validphys/results.py#L666 This could be linked to the wrong shape of groups_each_dataset_chi2 as constructed by https://github.com/NNPDF/nnpdf/blob/9840fc69dfcbcf25ebb30236916c4f1fc65a1785/validphys2/src/validphys/results.py#L663