Modify resclust outputs

[VGPReys]

To solve the two improvments addressed in issue #327:

1. Dict keys have been casted to string. Tests data have been modified too. While printed in log, dict is converted to string, and default ' character are replaced to ", making sure the json library will be able to loads(string) the data.

2. While providing the --output=path/to/results argument, the user can now add a path where to generate the Clutering results. An additional test function was added to make sure the output is indeeed created while providing the argument.