Formal verification (certora)

[MathisGD]

Formal verification (#56) of:

• Math.sol ✅
• CompoundMath.sol ✅
• WadRayMath.sol ✅
• PercentageMath.sol ✅

To do:

• [x] adapt README
• [x] add it to the CI

[Rubilmax]

certora is cool