expose prepareExpression and change the name to match other functions

[cdonnelly1992]

Exposing prepareExpression would allow someone to run the parse command by hand (prepare, tokenize, toRPN, parseRPN) and also make changes to the tokens/RPN if needed.

I'm pretty sure this covers everything needed to expose prepare_expression. Based on the other methods in the parser, I renamed prepare_expression to prepareExpression.