FLUID-5024: Add value transformation hook to UIO modelRelay

@amb26, could you have a look at this? It is my attempt at a very simplified version of the value transformation functionality in the modelRelay component that used to be part of the video player. You can see the video player's use of the new functionality in my video player branch upgrading the video player to the latest Infusion: https://github.com/acheetham/videoPlayer/blob/VP-297/js/MediaSettings.js