This PR does two things:

1. Makes the existing Lerp function constexpr and marks it with noexcept
2. Proposes a Lerp overload that takes a custom easing function. This easing function can easily be used to achieve different effects, such as bounce-in/bounce-out, ease-in/ease-out, etc. Overload can work with large types (vectors and matrices with properly defined operators)

An example easing function (bounce out) can look like this:

constexpr double BounceOut(double t) noexcept // Piecewise quadratic function to create a bouncing effect.
{
	if(t < 4.0 / 11.0)
		return (121.0 * t * t) / 16.0;
	if(t < 8.0 / 11.0)
		return (363.0 / 40.0 * t * t) - (99.0 / 10.0 * t) + 17.0 / 5.0;
	if(t < 9.0 / 10.0)
		return (4356.0 / 361.0 * t * t) - (35442.0 / 1805.0 * t) + 16061.0 / 1805.0;
	return (54.0 / 5.0 * t * t) - (513.0 / 25.0 * t) + 268.0 / 25.0;
}

This patch does and should not break anything.

P.s.: While the C++17 standard implicity assumes that a constexpr function is hinting inline, I explicitly added them for clarity.

Please review.