Issue a warning G4OpticalPhysics is not compatible with materials

[nkrah]

Issue a warning if G4OpticalPhysics is activated and materials are present in the simulation for which no optical properties are provided.

This would provide the user with helpful information. Reason: if an optical photon enters a volume with a material for which G4 does not have any optical properties, G4 cannot track the photon. The user should be made aware of this.

Note: Check how G4 handles the above case in detail in the stepping action. Does G4 kill the photon? In which part of the step - pre, post?