Add GlobalMaxPool3D layer

[mkaze]

Currently, the support for GlobalMaxPool3D is missing and it would be great if we could add support for this layer as well. The desired PR addressing this issue should include:

• Implementation of layer class named as GlobalMaxPool3D (you can take inspiration from implementation of GlobalMaxPool1D as reference)
• Documentation of layer.
• Test cases
• 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 this layer. 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 simply assume the channels are always in the last dimension.

[zaleslaw]

So, are you going to create PR by yourself or we will mark it with the "good first issue"?