feature: Refactor Layers plugin to integrate multimodality more easily

[samysung]

Pitch:

Odeon must be able to integrate any type of input data (modality agnostic), with potentially different read/write engines. All this while presenting a simple Api that hides the complexity. This is the role of the Layers Api.
To be able to do that, we need to refactor the code from layers, with implicit changes to the data internal plugin.

We need today, at least a DataHandler class to interface layers with the dataframe, a LayerCollection class to handle multiple layers in a duck typing way, and to start by making an Raster Layer and a Mask Layer.