Check for f2py when running MG reweighting

Hannes Mildnerrequested to merge
hmildner/athena:21.6-MG-rwgt-f2py-fix into 21.6

The MadGraph reweighting module requires f2py. If f2py is not available (which seems not to be the case, at least in some configurations on the grid) it fails quietly. So we now throw an error when reweighting is requested but f2py is not available. See also AGENE-1771 .

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