Add incident angle dependence to DIRC mirror reflectivity

[jrstevenjlab]

At today's DIRC meeting we discussed the need for applying an incident angle dependent reflectivity for the DIRC mirrors to match the data we've collected for the borosilicate/UV enhanced mirrors with an air gap. Currently a single reflectivity vs wavelength for each mirror is provide in the material description in hdds.

Adding this issue to the stack so we don't lose track of it and mentioning @rdom, since he had some suggestions on how to include this in the SteppingAction. And as a reminder to myself that the normal vector to the mirrors is computed already in the hit processing here

HDGeant4/src/GlueXSensitiveDetectorDIRC.cc

Line 191 in 49f42c3

G4ThreeVector localNormal = (iNav[hNavId])->GetLocalExitNormal(&valid);