[FIX] fields: in to_column(), returning self.column is generally not correct

[FIX] fields: in to_column(), returning self.column is generally not correct

Consider the following example:

    class Foo(models.Model):
        _name = 'foo'
        _columns = {
            'state': fields.selection([('a', 'A')]),
        }

    class Bar(models.Model):
        _inherit = 'foo'
        state = fields.Selection(selection_add=[('b', 'B')])

The attribute 'column' of the field does not have the full selection list,
therefore the column object cannot not be reused, even a copy of it.  The
solution is to systematically recreate the column from the field's final
specification, except for function fields that have no sensible way for being
recreated.