Make lookup for LHE headers case-insentive

For the moment, LHE headers are retrieved using case-sensitive string comparison. It makes all LHE file processed by MadSpin invalid, because until today, MadSpin converted all headers to lowercase (MGProcCard to mgproccard, etc.) (see https://bugs.launchpad.net/mg5amcnlo/+bug/1325955 )

This PR change the way we look for LHE headers for case-insensitive string comparisons.

It'd be great if you could also cherry-pick this on other CMSSW release, as I don't have much time to do it.