Use future API to support parallel AlignTraj

[kain88-de]

This is a proof of concept script how to use the futures api of dask to support the writing of results into a trajectory while still working on frames in parallel. It won't scale out a lot. It also assumes that the single_frame function returns a namedtuple with coordinates. The buffer can potentially blow up so use with care.

def dask_execution(single_frame, trajectory, client)
     # have writer in main process
     with Writer as w:
         futures = client.map(single_frame, trajectory)
         idx = 0
         for f in as_completed(futures):
             res = f.result()
             buf[res.frame] = res.xyz
             # check if we got a  result to write
             while True:
                 # write everything we can *in order*
                 if idx in buf:
                     w.write(buf[idx])
                     del buf[idx]
                     idx += 1
                # we can't write anything so break
                 else:
                     break
        # write leftover frames
         for i in sorted(buf):
             w.write(buf[i])