Add Cropping1D and Cropping3D layers

[zaleslaw]

Currently, the support for Cropping1D and Cropping3D are missed, and it would be great if we could add support for these layers. The desired PR addressing this issue should include:

• Implementation of layer class named as Cropping1D (you can take inspiration from the implementation of Cropping2D as reference)
• Implementation of layer class named as Cropping3D (you can take inspiration from the implementation of Cropping2D as reference)
• Common hierarchy of all Cropping layers with abstract cropping class with the common functionality
• Documentation of layer and all non-private methods
• JUnit tests in api module
• Support for export of layer to JSON (see ModelSaver.kt)
• Support for import of layer from JSON (see ModelLoader.kt)

Also, if needed, you can take a look at Keras documentation for Cropping1D and Cropping3D.

NOTE: for the moment, there is no need to add support for "data format" (i.e., channels last vs. channels first) in your PR; you can assume the channels are always in the last dimension.

P.S. If you want to take this ticket, please leave the comment below
P.P.S Read the Contributing Guidelines.

[mkaze]

I'll take this one.