Port detector related libraries from Gauss + documentation

Depending on the merging process in Gauss, MRs will be ported here:

needs !42 (merged) !27 (closed)

corresponding MR in Gauss: lhcb/Gauss!778 (merged)

• ExternalDetector lhcb/Gauss!750 (merged), test: external_cube.qmt
• MCCollector lhcb/Gauss!747 (merged)
• Export GDML with auxiliary information lhcb/Gauss!746 (merged)
• ParallelGeometry lhcb/Gauss!757 (merged), test: mixed_geometry.qmt (this also tests GDML export for both mass and parallel geo)

Additional TODO list:

• Port gauss_geo_planes.qmt to G-on-G
• Port ecal_collector.qmt to G-on-G
• Ported generic physics lists and geometry from Gauss, so that running Gaussino as a standalone application is less painful
• documentation