Train neural network interactively

[brollb]

Building on top of the addition of interactive editors, it would be nice to have an interactive editor which enabled users to train neural networks. Ideally, this would be included in https://github.com/deepforge-dev/deepforge-keras. However, it might be good to implement the initial version here as there is not yet support for extensions to define their own interactive editors.

[brollb]

This is also related to #1689 as this could replace the need to create a generic operation for training networks.