Graph Export Refactoring

Current state:

• a lot of overly specific exports
• mainly focused on compoundgraph and bipartite
• lacks of flexibility regarding attributes export
• a lot of duplicated code in apps
• gml as default and lacks of node export in app not adapted for workflow

Objective:

• an interface for apps exporting graphs
• parametrizable export
• harmonization and support for all graph types
• node table export in apps