Standardize the properties

We need to standardize the properties in this repo, for example:
- properties must be described with natspec
- the english description of the invariant must be present
- The mode must be described (internal /external / isolated / function override / model testing) (https://secure-contracts.com/program-analysis/echidna/basic/common-testing-approaches.html#partial-testing)

We should also agree on a structure to group the properties in the file, and how to document the sections (ex https://github.com/crytic/properties/blob/c25de701cc0e0def1bb175f024f7e7e0cb6f1575/contracts/Math/ABDKMath64x64/ABDKMath64x64PropertyTests.sol#L379-L384)

Once we have defined the rules, we should apply them on the repo, and integrate them in the contribution guidelines