Exposure slider does not change scene/model brightness

[blundellj]

Steps to reproduce:

• Open a scene in Voyager Story
• Select the Scene node using the Navigator pane
• Select the Settings tab from the ribbon at the top of the page
• In the task pane go to View>Render>Exposure
• Side/change the Exposure value

Expected results:
The scene should change brightness relative to the Exposure value

Current results:
The Exposure value does not seem to do anything

[gjcope]

This is because exposure is a property of tone mapping which is turned off by default. I've added a field toggle it on and off with a hard-coded tone mapping type (https://64.github.io/tonemapping/#aces) on the back end.

[gjcope]

Fixed in #104