Updated and expanded documentation

[BrandonXLF]

Hello, I've made a few changes to expand and update the documentation. I also made a few changes to the code itself to reflect these changes.

Changes:

• Updated the documentation for Formula.constructor to reflect that there is no parentFormula parameter
• Added documentation MATH_CONSTANTS and functions without documentation
• Added more specific parameter types for buildExpressionTree
• Updated documentation to show that valueObj is optional for Formula.evaluate
• Updated Formula.calc to reflect that valueObj is optional for Formula.evaluate
• Changed the return type of Formula.isOperator from a boolean or an array from String.match to just a boolean.