Change linear expression representation

[tbittar]

This PR aims at correcting two bugs :

• Representation of expressions with a lot of terms e.g. connection of an important number of components on the same port
• Ability to use parameter-dependent expressions in linear expressions coefficient and constants

The main logic is to de-recursify the representation of linear expression, sending most of the processing from the visitors to operator overloads. We keep AST for expressions involving only parameters (coefficient and constants in a linear expression)