Conform to scikit-learn requirements

[matteobecchi]

The OnionMulti class does not conform to the scikit requirements for the data input format.

At the moment, there are constraints on the size of the input array because it has to be a multiple of the number of dimension (because we are flattening the 3D array into a 2D one).

Because of this, the code does not pass the "check_estimator" tests. Currently I have no idea on how to solve this.

[matteobecchi]

The main issue of the previous comment has been resolved, simply raising errors or warnings in the right places in the code.

Now it remains just a single test which fails, I think a check that some trivial clustering is done correctly.