Initial work on issue 921: creating tests for FlattenerLayer.

I need some feedback on the following tests for FlattenerLayer:
1) I can't get the fprop to work on my toy example, what is wrong?
2) Do we need more tests on the "inner" FlattenerLayer to see if its other methods (e.g. get_monitoring_channels, get_weights, cost...) agree with what they are truly supposed to be? @dwf 

I checked, it is indeed a tuple, meaning that fprop_composite will have two inputs.