Move picking code into its own file

[lilleyse]

A majority of the code in Scene.js is now just for handling picking, made worse by #6934 and #7115. Picking code should be in its own file but still tightly integrated with Scene like View or SceneTransforms.

[lilleyse]

@IanLilleyT

This is the list of function that will move to the helper file. To avoid breaking changes Scene will still have the function signatures and documentation but the implementations will go in a separate file called Picking.js that is marked as @private like View.js.

• pick
• pickPositionWorldCoordinates
• pickPosition
• drillPick
• pickFromRay
• drillPickFromRay
• pickFromRayMostDetailed
• drillPickFromRayMostDetailed
• sampleHeight
• clampToHeight
• sampleHeightMostDetailed
• clampToHeightMostDetailed

Also make sure to move any local functions that these function call. There may be cases where these functions call local functions that are used by other areas of Scene. In that case convert the local functions to private member functions of Scene. Look at Scene.prototype.isVisible for an example of that (used by View.js).