Add mirrors to the Cherenkov channel & reduce memory usage

[SanghyunKo]

A couple of geometry updates

• Added mirrors (Aluminium reflector) to the Cherenkov channel (a la RD52), where reflection rate can be adjusted via the compact file.
• However, naive addition of mirrors exploded memory usage (4.6 GB → 6.8 GB), tried to reduce it by exploiting replicas (6.8 GB → 3.7 GB)

and misc. updates

• Updated the fastsim model for optical photon propagation
• Application of the yellow filter is now done in the stepping action

[SanghyunKo]

@kyHwangs FYI