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
It was added in 846ba3409b24bd7404f7f58d4674dd91ca5107f2 by Ondrej 2008! Maybe at that time it was ```Plot```. But yeah, it's strange that why it wasn't tested before. I'm getting doctest failure on the master too.