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 (merged). Either that one should be merged first, or closed and this one used instead.