Serialization definition

[ArnaudBuchholz]

Part of EPIC #271

As a developer, I want to flag a class definition with attributes to control how its serialization is done

Acceptance criteria:

• The attribute class is gpf.attributes.Serializable
• A structure will summarize all properties that can be set: name, type, required
• The list of known types is defined inside an enumeration (it contains string, number and datetime)