Revert ev_op_max_order to integer

[felixhekhorn]

• during the developments for QED ev_op_max_order has become a tuple (QCD, QED), because we thought we needed it
• recall that ev_op_max_order is the maximum perturbative order in which we expand the $U$ matrices for "perturbative-exact" - see also https://eko.readthedocs.io/en/latest/theory/DGLAP.html#id7
• however, later we found out that there is no "perturbative-exact" solution for QED, but only the iterated one - see QED paper

Question: do we want to change ev_op_max_order back to just an integer? @tgiani @giacomomagni

This would be of course a breaking change (and thus directly related to #417) and to make it even more explicit we could also rename it to ev_op_max_order_qcd

[giacomomagni]

Since we need to do a compatibility layout in any case we can add this additional conversion...