Function multiply

Added these methods, and appropriate test cases:
- MathFunction.equals
- MathFunction.multiply( MathFunction )
- Term.multiply( Term )

This is part of the work I'm doing for Issue #52. I put this enhancement on a separate branch because it could be viewed as a distinct enhancement. Also, it can get merged sooner.

I based this on the 'issue54' branch, so I recommend merging this PR to 'master' only after merging #62.