Rewrite and verify LSTM implementation

[julianhoever]

The LSTM implementation is currently completely broken. The HardTanh and HardSigmoid activations used in the LSTM cell template require bufferless components, which we do not support at the moment. We also need to verify that the LSTM cell and LSTM network templates are filled correctly and that the tests for these components are sufficient to ensure correctness.

[julianhoever]

I deleted the current broken LSTM implementation in commit c524ca20cc49333007c4e0bbfa167912580e5c01. We have to reimplement it when working on this issue.

[glencoe]

lstm implementation has been added again in current develop branch