[enhancement] Move dummy models and costs to testing

[TNick]

I propose:

• creating a new module in testing (model.py) with DummyModel, SoftmaxModel and TopoSoftmaxModel
• moving SupervisedDummyCost and DummyCost to cost.py

Those are all located in test_sgd.py and can't be imported.

That's because I would like to avoid duplicating the code in pyl2extra.

Any objections?

[bouthilx]

Looks like a good idea. I don't have any objection.