Not all classes document invariants

[louis-langholtz]

Expected/Desired Behavior or Experience:

Every PlayRho library class-class documents one or more class invariants or — if it doesn't have any invariants — is refactored to being a struct-class.

Actual Behavior:

Lots of PlayRho library class classes are currently defined without documenting one or more invariants.

Steps to Reproduce the Actual Behavior:

Search for class definitions in the PlayRho library and see which definitions don't document any class invariants.

[louis-langholtz]

I still like this as a goal but not enough to hold up release 1.0. Changing its milestone to 1.0+ (after 1.0). Part of the problem I'm having with this issue is one of scale. Maybe it'd be better for this to be a project rather than a single issue.

[louis-langholtz]

Converted this intent into a Classic Project: Document class invariants.