Compatibility with scikit-learn object API

[iwiwi]

The estimators do not seem to have full compatibility with scikit-learn object API, which is described here.

For example, fastFM.sgd.FMRegression does not store l2_reg in __init__. This prevents sklearn.base.clone to clone an estimator, as shown in the image below. Due to this problem, currently we cannot use some of sklearn's useful utilities such as GridSearchCV.

As recommended in sklearn's coding guide, we might want to test the compatibility by using check_estimator, and modify the estimators to pass the test.

[ibayer]

Full API compatibility is certainly something we are aiming for. Could you list what else is missing?

[ibayer]

Issue #49 is also related to incomplete API compatibility.