Definition of particle masses

We should have one common place where particle masses are defined and those should be used in all selection lines. Currently in Allen the pion and muon masses are defined here. Some lines define their own masses (such as here) or define them as properties (such as here).

I suggest to add a header file in main/include similar to the unit definitions and to use masses defined there in all of HLT1.

Ideally, these should be the same definitions as used in HLT2 and further downstream.