Cherenkov photon detector

Added a "mirror" photon counter and pmt photon counter. Both produce hits in the output hits collection and only detect optical photons. They can be sorted out in digitization.

src/ConceptDetectors/SoLID/compact/gps.mac

@@ -6,7 +6,7 @@
 /gps/number 1
 
 /gps/ene/type Gauss
-/gps/ene/mono  3.0 GeV
+/gps/ene/mono  5.0 GeV
 /gps/ene/sigma 2.0 GeV
 
 /gps/pos/type Volume
@@ -16,7 +16,7 @@
 /gps/pos/halfz  0.01 cm
 /gps/position 0 0 0
 
-#/gps/direction 1 0 0 
+#/gps/direction 0 0.1 1 
 /gps/ang/type iso
 
 /run/beamOn 100