Clamp coefficient of restitution between 0 and 1 and improve restitution docs

[Jondolf]

Clamps each Restitution coefficient to be between 0 and 1, adds the associated constants Restitution::PERFECTLY_INELASTIC and Restitution::PERFECTLY_ELASTIC, and significantly improves the restitution docs. This closes #114 and #115.

The clamping avoids issues with unrealistic and explosive behavior when users try to use values that are negative or over 1, which makes things more user-friendly and realistic. For the rare cases where you want bodies to gain kinetic energy on contact, you should implement it manually to work for your specific case in a stable manner.

The main caveat is that you can no longer multiply e.g. 0.5 and 2.0 to get 1.0 when combining coefficients, since 2.0 gets clamped to 1.0, which results in a final coefficient of 0.5. However, I find this clamping to be more explicit since it's clamping the actual component values and not internally clamping just the combined coefficient, and engines like Unity seem to be doing it like this as well. It also allows us to perform the clamping at initialization and not individually for each contact.