Add support for `FeatureUnion.transformer_weights` attribute

[vruusmann]

Somehow this attribute has gone unnoticed (maybe a recent addition?).

Two-step plan:

1. The converter should be(come) aware of it, and fail with an IllegalArgumentException("Attribute FeatureUnion.transformer_weights is not supported") if it is initialized with non-null/non-unit weights. This is to prevent irreproducible transformation results between Scikit-Learn and (J)PMML.
2. Add full support.