save project in YAML instead of xls

[afourmy]

the model is to complex to be stored in an excel (per-AS node properties, etc)
it must remain possible to import object and object properties with excel, but the saving of a project must be done in .yml format

[afourmy]

Saving of nodes and links: done.
Remaining parts:

• saving of AS
• saving of interfaces
• saving of shapes