[WIP] Experiment multiple backends

[YannCabanes]

[YannCabanes]

Hello @rtavenar, I have started a PR about multiple backends. To begin with, we only focus on numpy and pytorch. I have created a new file named tslearn/experiment_multiple_backends.py in which I have created several backend classes and a test classifier. The objective for now is the create a backend structure and a classifier structure such that the classifiers (and other machine learning algorithms) of tslearn will adapt to the backend of the data provided by the user. Once we have a better idea of the structure of what we want for the backends' structure, I plan to split the experiment_multiple_backends.py file into several python files for each backend and a test file.