Correct MDGXS.print_xs method

[nelsonag]

When creating and evaluating #2042, it was identified that the MDGXS class raises both TypeError and IndexErrors when printing any of the delayed MGXS types. Both are due to the shapes of the average cross section and their relative errors (average_xs and rel_err_xs variables, respectively) not being what is expected. The issue could exist in the print_xs methods of MDGXS and MatrixMDGXS, but also could be due to an erroneously implemented dont_squeeze attributes of these classes (and inheriting classes). Further inspection is required.