Improve serialization

I have tried a first approach to define classes and services for serialization.
All important changes are in file [core/serialization.py](https://github.com/gbaty/openalea/blob/improve_serialization/core/src/core/serialization.py):
  - ISaver
  - ILoader
  - ISerializer
  - IDeserializer

@pradal, It is just a first draft, this pull-request is mainly a "review-request" for Iclass API.

Associated services (get_saver, get_loader, load, save, ...) don't have changed yet.
It will be done in nex pull-request.