PixelGeoModel: Cache DBM volume calculation.

Cache the volume calculated in DBM_Telescope::Build(). It's always the same, and finding the volume of a subtracted shape is very expensive --- GeoModel does it by doing a MC integration within the bounding box with a hardcoded number of sample points of 1e6. This is particularly slow in a debug build, as we have to do a bunch of eigen calculations for each MC point.

This reduces the initialization time in debug builds by several minutes.

(Would be even better of course to do this calculation analytically!)