Major refactor

Major changes to clean up the API.

Combines StackFourier and StackConformal into a single class Stack with runtime-separation of the conformal or fourier logic. This does not seem to affect performance in any measurable way. It allows to simplify the python API, since bindings for only 1 instead of 2 classes are needed.

• Model now inherits from Stack to expose all functions with bindings.
• Modefinder no longer inherits from Model and uses composition instead.
• found modes will be normalized in units of inverse wavelength
• some smaller changes