postProcessVolume lerping and transition configurations

[ccpHenry]

Adding some fading options to EveChildPostProcessVolume.

this will help limit/remove abrubt PP changes when changing environments (f. ex. warping into dungeons) when using these volumes as well as intensity scaling for smooth artistControlled transitions

Changes

• new value: enabled (default true), turns the volume's effect off without deleting it. It fades out over minimumTransitionTime.
• intensityMultiplier (default 1.0): scales the final intensity, so you can animate or tune it from curves and controllers.
• minimumTransitionTime (default 0, which means instant): the number of seconds to fade between the minimum and maximum intensity. Intensity moves toward the target each frame. The fade speed scales up with the size of the change and with the multiplier, so large jumps don't lag behind.
• Intensity is now calculated as a target value and then blended in, instead of being written directly. With the defaults, behaviour is the same as before.

[filipppavlov]

Please provide a description to the PR