StiXxxDetectorBuilder: Removed ionization from StiMaterial constructor as it is not used anywhere including energy loss calculations
StiXxxDetectorBuilder: Removed definition and addition of "hybrid" material because it is not used anywhere
StiXxxDetectorBuilder: Refactored gas material definition

For the gas surrounding the detector we use air properties as defined in global
geometry manager. If the AIR material is not available we use default hardcoded
properties
StiXxxDetectorBuilder: Refactored silicon material definition for sensitive layers

Only silicon material is used in construction of sensitive layers. As with the
gas we first try to use the material definition in the global geometry manager.
If the SILICON material is not available we use default hardcoded properties