Update TargettedPixelGroup to align with Raysect 0.9 naming #492
Description
Raysect 0.9 fixed a typo in all occurrences of word targetted to targeted. Including names of classes and methods. The minimal changes required for adoption of the new Raysect version was handled in #486, excluding group observer classes formerly inheriting from TargettedPixel.
Based on discussion regarding the required changes I would like to update naming to TargetedPixelGroup and create a legacy subclass with the old names and deprecation warning. The legacy class should be kept until 2.0 is released.