Standardize the properties

[montyly]

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

properties/contracts/Math/ABDKMath64x64/ABDKMath64x64PropertyTests.sol

Lines 379 to 384 in c25de70

	/* ================================================================
	TESTS FOR FUNCTION sub()
	================================================================ */

)

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