Add ability to set speed of animations

[johnwhh]

Proposition

In the current version, there is no way to pause, reverse, or change the speed of animations on a ModelNode. This enhancement would bring each of those capabilities.

Changes

• A speed variable has been added to the PlayingAnimation data class to give each animation its own speed. The speed can be changed while the animation is playing to dynamic change the rate at which the animation is played.

• Two functions have been added to assist in changing the speed of animations, both of which keep the precedent set by the playAnimation functions.

fun setAnimationSpeed(animationName: String, speed: Float)
fun setAnimationSpeed(animationIndex: Int, speed: Float)

• Each playAnimation function can now be passed a speed. Each one uses a speed of 1f by default.

• The logic for applying animations has been moved into its own private function called applyAnimations to make the onFrame call site cleaner.

• Added some missing documentation to the playAnimation functions.

Enhancements

To reverse an animation, use a negative speed:

playAnimation(0, speed = -1f) 

To pause an animation, use a speed of zero:

setAnimationSpeed(0, speed = 0f) 

[ThomasGorisse]

Thanks! That's clean code!

[MicahMeadows]

LGTM