Enable using percentage dose for isodose levels

[cpinter]

• [ ] Enable using percentages of prescribed dose as well as absolute values
• [ ] Use the Eclipse isodose colors as default (80% deep blue, 90% lighter warmer blue, 100% yellow, 105% orange, 110% red)

(this is a new task for subtasks in the closed ticket https://github.com/SlicerRt/SlicerRT/issues/18)