Add return on investment to math

[InukaWijerathna]

Description

Adds return_on_investment to the math/ directory.

Return on Investment (ROI) measures how profitable an investment is relative to its cost:

ROI = (Gain - Cost) / Cost × 100

What's included

• math/return_on_investment.cpp — implementation under namespace math, with Doxygen documentation, assert-based self-tests, and std::invalid_argument for non-positive cost

Checklist

• Follows the code style of the project (Doxygen comments, namespace math, assert-based tests, main with test runner)
• Throws std::invalid_argument for invalid input
• Self-contained with main() running all tests

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

[InukaWijerathna]

Closing this for now — needs more work on my end before it's ready.