Remove some nesting

Flatten the file structure some, and move and rename some things.

1) Rename the `core` subpackage to `local`
2) Move the `proxy.py` and `benchmark.py` files into `benchmarks`
3) Move `mpi/mpibase` to `mpiutils`, merge `mpi/error` into `error`, and remove the `mpi` subpackage
4) Move `random/nprand` to `local/random` and remove the `random` subpackage.  It was only one file (aside from its tests).
5) Some random PEP8.