Remove Automatic Evaluation

- [X] Stop auto eval for Xor
- [X] Stop auto eval for ITE
- [x] Stop auto eval for Not
- [X] to_nnf
- [x] is_nnf
- [X] is_literal for predicates
- [x] Tests
- [x] Documentation
There are some formatting and grammar issues here. The first line should be on its own. Reformat this as

```
Checks if expr is in Negation Normal Form.

A logical expression is in Negation Normal Form (NNF) if it contains only And, Or and Not, and Not is applied only to literals.

If ``simpify`` is True, checks if the result contains no redundant clauses. 
```