Add return on investment to math

[InukaWijerathna]

Description

Adds return_on_investment to the math module.

Return on Investment (ROI) is a straightforward but widely used financial formula — it tells you the percentage gain or loss on an investment relative to its cost:

ROI = (Gain - Cost) / Cost × 100

What's included

• algorithms/math/return_on_investment.py — typed implementation with full docstring, doctests, and ValueError for non-positive cost
• algorithms/math/__init__.py — export added

Checklist

• Follows the code style of the project (docstring with Args/Returns/Raises/Examples)
• Doctests for all cases including error paths
• Export added to __init__.py

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

[InukaWijerathna]

Hi! Just checking in to see if there's any update on this PR — happy to make any changes needed to get it merged. Thanks for your time!

[keon]

@InukaWijerathna Thanks for the contribution! can you rebase on the current main branch? I think there's a lot of unrelated commits added to this PR

[InukaWijerathna]

Done! I've rebased onto the current master branch — the PR now contains only the single commit adding the return on investment function to algorithms/maths/. (Also updated the path from math to maths to match the current repo structure.)

[keon]

it is such a simple code that I am not sure if it is worth packaging it as a library... sorry about that.