More flexibility in soft_sorting operations

In this release:

• it is now possible to parameterize the squashing function used by the sort.py module to squash/squeeze input values in [0,1]. The default behavior remains sigmoid of whitened values, but this can be now changed to any other approach, including using the values directly without squashing them (e.g. squashing_fun= lambda x : x).
• handling of keyword arguments kwargs is facilitated in the sort module. Before user was supposed to pass a dictionary, now kwargs are passed directly.