Add ThresholdedReLU layer

[zaleslaw]

We are missing some activation layers to support the export of models from Keras fully. One of them is the LeakyReLU layer.

Add an activation layer class, write documentation for it, write a test for it, try, if possible, create a small trainable network with it (in your own GitHub) and attach a link here in the comments.

The layer should be placed here

As a reference implementation, the ReLU activation layer could be used, but feel free to improve it!

Also, support for export and import of activation layer in JSON format should be added (see ModelLoader.kt and ModelSaver.kt)

A detailed description of the activation layer can be found here

[zaleslaw]

@mkaze, please write a comment here, I could assign a ticket for you

[mkaze]

@zaleslaw Sure! I forgot to leave a comment for this one before starting to work on it.