Rewrite optics.py

[BenjaminMidtvedt]

optics.py is currently overly complicated with a lot of code duplication. A rewrite should accomplish

• Move all volume padding and indexing out of the get method.
• Allow more granular control of the execution of the code. For example by moving sections into methods that can be overridden by subclasses.
• Make helper functions class methods.
• Allow direct control of which property to access from scatterers to create the volume.
• Include more systems, such as darkfield.