Ideas for fast interpolation

• Refactor interpolation methods from the current Enum + Dict to a proper class with a method
• Those classes have a "raw interpolate" method that doesn't take units
  • Cannot inherit from a @jitclass we could use composition instead
• Ephem takes an interpolation method instance and Ephem.sample calls the corresponding interpolate method
• We add a (private?) "raw sample" method without error checks that calls the "raw interpolate" method

Notes:

• Try to do this in a way that doesn't feel like a mega hack
• Will we need Ephem for the build_ephem_interpolant at all?