Akima interpolation

• Added a new interpolation option to RingOptions ("akima"), which uses the Akima1DInterpolator from scipy.
• Structural changes to RingOptions that move each interpolation method into a separate function to simplify reading the code.
• Added the annotations import from __future__ and started updating the type hinting.

Also includes changes that are in merge request !31. Either that one should be merged first, or closed and this one used instead.