Expression visitors : first implementation

[flomnes]

TODO

• [ ] Add missing object types
• [ ] Add missing visitors
• [x] Re-organize source files / split header/source, use the usual hierarchy with include/XXX/, define visitor / node types separately, etc.
• [x] Deep copy for nodes (copy operator/constructor is currently deleted because of std::unique_ptr)
• [ ] private children for nodes ?
• [ ] Use a generic type to avoid defining ~14 object types manually ?
• [ ] Sonar issues
• [ ] Separate tests (move existing calls to test, add more tests)

~~The following causes a double free because of two std::unique_ptr holding the same memory address~~

Parameter* p = new Parameter(1.);
Add(p, p);