Fix backwards compatibility for extraction of event_norm from MadGraphUtils

Description of bug

In !800 (merged) a change was introduced where Py:MadGraphUtils INFO "event_norm" was substituted with Py:MadGraphUtils Setting event_norm which only exists in more recent releases.

Moreover instead of reading the whole file, this test was harmonised with the others, i.e. the relevant information was extracted once from the log and stored in a dictionary. Previously the whole log was read for the madgraph checks, which effectively prevented us from applying madgraph checks for shower-only jobs, which is what is causing the crash reported in ATLMCPROD-8957

We fix both issues here

Changes introduced

Re-introduce "event_norm" in the generator dictionary and use it as a fall-back solution when Setting event_norm is not found. Also check against the presence of the dictionary entry before extracting its value which should prevent us from applying the test in e.g. shower-only jobs.

Tests

logParser runs successfully with logs from ATLMCPROD-8957 (shower only) as well as with MG logs from OTF jobs (where the old "event_norm" printout was used). Also works with logs from !800 (merged) (where the new Setting event_norm printout is used)

Issues resolved

Closes #