Add EGADS attenuation measurements

Add a new water material with the attentuation measurements from EGADS
provided by Pierce Weatherly.

N.B. The refractive index wasn't measured for this water, so I've copied
`RINDEX` from the `HeavyWater` material in `SNO/OPTICS_SNO.ratdb`.

This has the optics, but the Gd-related physics needs to be checked. I seem
to recall Braidwood RAT doing something special to get Geant4 to handle
this correctly.

@nbarros I had to add Gd and S to `ELEMENTS.ratdb`, so the fall-back to
the NIST elements table must not be working right. Could you look into it?