Remove incorrect use of const in Event.h

[ChrisJChang]

I accidentally introduced this in my recent heputils bug fixes. This was causing the Mac CIs to fail because they were getting confused about an overload.

@agbuckley , this will require updating on the main heputils repo as well. Happy to make another PR if you would like.

[ChrisJChang]

The CI jobs will not fully succeed due to some other CI issues that we are in the process of solving ( the Mac CI jobs fail because of issues with rivet/contur.), but they can build gambit, which is further than the master branch gets.

[agbuckley]

I accidentally introduced this in my recent heputils bug fixes. This was causing the Mac CIs to fail because they were getting confused about an overload.

@agbuckley , this will require updating on the main heputils repo as well. Happy to make another PR if you would like.

Thanks @ChrisJChang ! I've directly applied this and the _cseqs copying in HepUtils upstream, and made a 1.4.2 release tag. No extra PRs needed :-)

[ChrisJChang]

Thanks Andy :)

[anderkve]

Looks like the Ubuntu CI job is just failing due to this known issue: #463

[anderkve]

Since this is a tiny fix and the CI jobs are just failing due to issues we are working on elsewhere, I'll merge this to master now. Then we can get it into e.g. #462 and make further progress there.