RaoResult writer API

[pet-mit]

Describe the current behavior

RaoResult can be exported to json, core cne, or swe cne. Three APIs exist for those purposes.

Describe the expected behavior

Unify the API by creating one method : raoResult.write

Describe the motivation

cleaner API, ease to add new exporters, ease to develop python api

Extra Information

• The method has the following arguments : CracCreationContext, Properties (that contains all specific information needed for specific exporters)
• The CNE exporters shall no longer need the Network : some preparatory work might be needed (for instance, adding information to the CracCreationContext)
• If it's useful, we can add a second method that uses a Crac instead of CracCreationContext
• If it's a lot of work, try to split it up in multiple Pull Requests