Use cleaner plugins include

[bartosik-hep]

Currently MuColl_v1.1 has plugins defined in the root geometry file, leading to code duplication in the reduced versions of the same geometry.
An issue preventing plugin definitions from being included to a root file has been recently fixed in DD4hep, but not yet included in the latest release v01-25-01.
Once a new DD4hep release comes out, a new MuColl_v1.2 version should be added that uses new <include> functionality to define plugins only once.