Port Beam-Gas options to Sim11 on master
@gcorti @admorris C++ files should be ready to be used almost as is, but it needs porting of the configuration
The following discussion from !985 (merged) should be addressed:
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@mimazure started a discussion: (+2 comments) This has been removed in Sim11.
Added this CMakeLists just to keep the track of !857 (merged) with beam-gas collisons.