Add ReturnOnInvestment to math

[InukaWijerathna]

Description

Adds ReturnOnInvestment to the math package.

ROI is a fundamental financial metric — it quantifies how profitable an investment is relative to its cost as a simple percentage:

ROI = (Gain - Cost) / Cost × 100

What's included

• ReturnOnInvestment.java — clean implementation with Javadoc, throws IllegalArgumentException for non-positive cost, and a main() with sample outputs

Checklist

• Matches the existing code style (package declaration, Javadoc, main() demo)
• Throws IllegalArgumentException for invalid input
• Sample outputs in main() showing expected values

Reference: https://www.investopedia.com/terms/r/returnoninvestment.asp