Some work on importance weights.

- Default parameter for weights is now False, not 1.
- MLPs also accept weights.
- Normal RNNs work with weights.
- Added tests for normal RNNs, FD-RNNs and mops.
- Added documentation.
Some more changes to the importance weights.

- Named ``imp_weight`` everywhere.
- Models have set it to ``False`` per default.
made numerous tests faster