Move attribute formula_constructor to column.
Move attribute _func to column.
Always create a new state, to avoid multithread errors.
Rename method sum_by_roles to sum_by_entity.
Rework simulation initialization.
Simplify Holder.compute.
Cache legislation JSON instead of parsing it, each time a tax benefit system is created.
Add method to compute the dependencies of columns.
Add a version

@ervaiart: this is needed bu openfisca-qt but should we put it here ?