dRICH: update mirror parameterization

**Is your feature request related to a problem? Please describe.**
Currently we have 2 parameterizations available, both using acrylic-substrate mirrors:
1. flat 90% reflectivity (implemented here in `epic`)
2. data points from measurements of AlMgF2 coated on thermally shaped acrylic sheets, from AJRP, 10/01/2012; this is from [Evaristo's common optics class](https://github.com/eic/drich-dev/blob/cfe8410ca2bdadba8d27bf236995bb71484151e7/src/g4dRIChOptics.hh); Silvia mentions that this performance is not nearly as good as we will be able to achieve, and will send us a paper with a better mirror

**Describe the solution you'd like**
Implement the parameterization from Silvia