Add ObservationCubic to support volumetric observatories

It would be good to have a ObservationCubic that works similar to the existing ObservationPlane.

The ObservationCubic can store particles arriving at the Cubic surface in a output file. And then, user can send those particles to further detector simulations.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information