Interface for Custom C++

[Laurie Nevay]

There already now exists an interface for customising BDSIM through a user's C++ application compiled against BDSIM. However, several new interfaces could be added. This issue is a holding ground for them.

• Custom user component done

• Completely custom physics list done

  1. maintain compatibility with BDSIM's requirements

• Custom material registration

• External c++ event generator / user primary generator

  1. maintain compatibility with local coordinates - ie fill zeros if not applicable
  2. set Geant4's user vertex information on their vertex that BDSIM uses

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[Laurie Nevay]

Implemented custom physics list interface in C++.

[Laurie Nevay]

Add custom material registration to the list.