generate initial random configs in parallel

[bernstei]

With large numbers of walkers and atoms, generating the initial configs takes a significant amount of time. It would be helpful if, when possible, these were generated in parallel rather than all generated on the root MPI task and sent to each other MPI task.

The new config generator would need to receive the local rng, and return a logical flag indicating whether the loop should be on the root task over all configs, or a little loop on the generator on each task in parallel.