TOKENIZE!

Changed course again, away from tree structure, and onto a tokenizer.
This breaks up the expression nicely into individual components of an
expression. I’ve also written a function to change the raw tokens into
RPN which will hopefully be easier to evaluate. (Key word: hopefully.
Still haven’t written evaluate)

P.S. More comments were made!
Evaluator works!

I’ve changed god to a queue, rather than a stack which is what I now
realize I want for evaluation. Fixed part of tokenizer that would
remove ALL instances of certain substrings, but now doesn’t do that. :)

Fixed the issue of leaving the operator stack empty.

More fixes to come:
This is not yet an integrator. I’m playing around with different
numerical methods of integration rather than just brute force Reimann
Sum.
There are definitely some places that could be made more efficient.
Also need to make more comments.