[Video] Add AveragePooling3D layer

[zaleslaw]

We are missing some layers to support the export of models from Keras fully. One of them is the AveragePooling3D layer. A detailed description of the layer can be found here.

• add layer class to the package using AvgPool2D as a reference implementation
• add documentation
• add test
• add export function (see ModelSaver.kt)
• add import function (see ModelLoader.kt)
• (optional) create an example

[mkaze]

I'll take this one.