Data model definition (*schema*)

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The schema is the core piece of a `CubicWeb` application as it defines

the data model handled. It is based on entities types already defined

in the `CubicWeb` standard library and others, more specific, we would 

expect to find in one or more Python files under the `schema` directory.

At this point, it is important to make clear the difference between

relation type and relation definition: a relation type is only a relation

name with potentially other additionnal properties (see XXXX), whereas a 

relation definition is a complete triplet 

"<subject entity type> <relation type> <object entity type>". 

A relation type could have been implied if none is related to a 

relation definition of the schema.