Expression Refactor Todo list

[rocky]

• [ ] Turn some BaseElement into @attribute get_attributes Same thing for get_elements, get_head, etc?
• [ ] Move is_inexect out of BaseElement
• [ ] Move get_float_value() out of BaseElement
• [ ] Move do_format and format is out of BaseElement

[mmatera]

My guess is that

• Expression is the only thing that should have elements.
• get_attributes has a parameter definitions, so it can not be put as an @attribute. Also, I wonder how far we want to keep methods that depend on Evaluation or Definition objects to be member functions.
• get_float_value is something that makes sense just for Integer, Rational, MachineReal, and PrecisionReal, in the best case.