Give more information if an exception is raised due to a type problem
Some classes to manage the variable frontend : Domain

A DomainNode class is introduced to represent both an Alternate (Alt) and
an Aggregate (Agg).

The domain factory provides a static method to normalize and clean
the user specification of the domain.
Introduce a factory to create a clean variable based on a domain.
New version of Messages with a simple AbstractMessage and RawMessage (WIP).
Introduce an abstract method to generate messages based on AbstractField definition.
Introduce the generate method and manage the frontend between the field domain and variable.
Add some import resolutions.
Some work on the Symbol implementation.
Introduce some tests.