Update rql.base.Node class:
- define self.parent as a TypeVar
- define self.children as a List of TypeVar
- Those 2 TypeVar are bound, i.e: they can be BaseNode or any children of BaseNode

Update rql.stmts.Union class:
- add a new attribute, i.e:  `children: List["rql.stmts.Select"]`

Update rql.RQLHelper simplify & _simplify methods:
- remove call to cast method, no more need to cast from BaseNode to Select

Also:
- Try to harmonize type signature for get_description methods
- Remove now obsolete comments related to type hints
- type vconsts as a List of Constant