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 

Since `fprop_composite` has two inputs, you would have to either pass it two subtensors of `X_data`, something like `fprop_composite(X_data[:, :5], X_data[:, 5:])`, or have two data matrices for `X` in the first place.