Concurrent Futures style multiprocessing context manager

[jadball]

We need a concurrent futures interface inside of ImageD11 that we can import as some kind of Pool, so that you can do this:

with ImageD11.futures.pool(kind='slurm','mpi','threadpool','multiprocessing') as Pool:
    Pool.map(big_scary_function, inputs)

kind specifies a backend (like matplotlib backends)
Could be read from an environment variable by default