Merge `optical_materials.xml` into `materials.xml`

**Is your feature request related to a problem? Please describe.**
There are currently two sets of material definitions, which will lead to overlapping entries (e.g. PbWO4 is moving in that direction with multiple definitions shared between optical and non-optical).
- `materials.xml` is a definition file for materials using the GDML schema.
- `optical_materials.xml` uses the lccdd schema.

There is no fundamental reasons to have two files, other than (possibly) readability due to optical property matrices (a possible split which does not lead to duplication could therefore be to keep those optical properties in a separate file).

Furthermore, `optical_materials.xml` contains more than materials, i.e. it contains optical surfaces.

**Describe the solution you'd like**
- A single `material_properties.xml` file with tables of materials.
- A single `materials.xml` file with all material definitions, which includes `elements.xml` and `material_properties.xml`.

Optical surface properties may need to move to dedicated subsystem files where they can be maintained, or need to be made generic. E.g. `MirrorSurface_DRICH` could be renamed `Anomet_Miro_Silver` or whatever will be used.

**Describe alternatives you've considered**
Maintain the status quo...

**Additional context**
This could be combined with #47.