ZMaxPlane

needs !37 (merged)

@gcorti @kreps @adavis

• port lhcb/Gauss!738 (merged) to Gaussino

Additional:

• zMaxForStoring default value changed to 10 km (it was 12280 mm) as Gaussino should be an experiment independent project and all the LHCb default options should be set by Gauss-on-Gaussino. I've added a warning message in case somebody forgets about setting that value.
• apart from the unit test test_ZMaxTilt.qmt that is already available in Gauss, I've added one more integration test zMax_external_geo.qmt that tests the zMaxPlane and other TruthFlaggingTrackOptions with the external geometry. The idea is to shoot two 100 MeV photons in opposite directions along the y-axis. There are 2 external sensitive detectors made out of lead: NorthPlane at (0, 100m, 0) and SouthPlane at (0, -100m, 0). ZMaxPlane is set at the origin with a 45 degrees tilt. As a result, the NorthPlane should store information about all particles and tracks created in a shower and the SouthPlane should store only the information about the incoming photon.