[WIP] Add multi layer adaptation possibility + add flattening of each feature outputs

[tgnassou]

This PR allows the user to put several layer names to adapt to different network levels, as done in several papers.

This PR also forces the features of each layer to be 2D, answering Issue #177.

It adds a lot for loops in the code; I don't see another way to do it; let me know what you think.