Add interface for common RNGs

[sethrj]

The RNG in random/ was based (on my suggestion) on what Shift does, using Nvidia-provided GPUs. To test RNG-based code on CPU as well, and if we want to build on HIP or another platform that doesn't provide curng adapters, we'll need something more portable. @whokion

• Move RNG classes in random/ into random/cuda (see Add CUDA random number generator #2)
• Add similar interfaces to one or more appropriate vecmath RNGs -- either porting vecmath code, or (probably better as a first step) just providing an interface to the vecmath

As the physics develops we'll want to add additional distributions (besides "uniform"). @amandalund you might be the best person to add a random/distributions/IsotropicDistribution class ported from the Nemesis/numerics/Uniform_Unit_Vector_Distribution class. Would you be up for that or shall I?

• Sample isotropic direction on device

[amandalund]

Sure, I'm on it.

[sethrj]

The decision was made to tweak the CUDA XORWOW RNG to be host-compatible rather than reimplement VecMath RNGs in Celeritas, since they were optimized for SIMD vector units.