Potential optimizations

• Returning early from interact kernels before constructing thread views (@pcanal)
• Pulling RNG state into local memory in RNGEngine constructor, then writing back to global memory in the RNGEngine destructor
• Rearrange memory layout of data to have more struct-of-array accesses (e.g. have MaterialTrackView::element_scratch aligned and strided by number of tracks, change particle data to have energy and def_id as separate contiguous arrays)
• Possibly allow inter-thread cooperation, refactoring track views and such so that they have null-ops for inactivate threads (except when being cooperative with other threads) rather than just returning early