Add Watchman Geometry

Adds RAT-PAC version of the geometry originally implemented in Baccarat. 

For basic use add these lines to the top of your macro file:
```
/rat/db/set DETECTOR experiment "Watchman"
/rat/db/set DETECTOR geo_file "Watchman/Watchman.geo"
```
I've mentioned these before, but possible issues:
- Inner PMTs currently overlap the shield (```G4Polyhedra``` components) in a way that does not break tracking code (but I've only done limited testing). The photocathode is entirely inside the shield and the back of the PMT is outside (c.f. Baccarat's geometry). If this is undesirable I can punch holes in the shield while adding backs to the PMT, but that only seemed to slow the geometry down.
- Neutron capture properties for Gd-doped water need validating. As a first pass I copied the properties that seemed relevant from RAT-PAC's ```water``` material. I defined the ```doped_water``` material in ```./data/Watchman/MATERIALS_Watchman.ratdb```