Retrieving results from resclust output

[VGPReys]

The parsing of the resclust output is not straightforward, here I descovered two little improvements that I put in this issue:

1) The loading of the dictionary printed in the Standard output is not possible using json.loads() as keys are integer. Modifying keys to string should solve the problem.

2) Possibility write the content of the clusters, hence enabling reading of this file instead of having to parse the StdOut.
Adding a --output option to command line argument should do the trick.