Lazy Features

[timkpaine]

• [x] allow configuration of the floating point threshhold for "changed" locally and globally
• [ ] add support for serialization
• [ ] allow for distributed calculation
• [x] Reuse expression in additional derived classes (https://stackoverflow.com/questions/41678621/how-to-rewrite-an-expression-in-terms-of-an-other-expression-in-sympy)