DD4hep and Material Mapping

Closes #642 (closed)

This merge request synchronises the material mapping and DD4hep geometry building with the recently updated ActsExtension.

It introduces a xml2Acts translation for ProtoSurfaceMaterial in order to run the SurfaceMaterialMapper

It also fixes:

• translation of proto material through geometry building for boundary surfaces
• empty hit assignment errors (i.e. material rays that hit a surface but don't get material assigned) during material mapping. This required a a new voidAverage() method to the accumulation classes that counts empty hits.