Changed the build-point of the feature factory. It works now together with the LBN as intended
As described in Issue #1, the keras layer does not work as intended.
The feature factory caches the tensors and thus any new call leaves the connection to the first input tensor untouched.
no_cache is only an internal option, I rebuild the build/call function, in a way that the feature factory is build at call and not init.
Furthermore, registering features now maintains a list of features, that is build in the build function. The docs need to be updated in an appropriate way, so that it is clear when
available_features is updated.