Channel interpolations

In the Channel class, one can choose to use interpolation for better efficiency by calling the void UseInterpolation(bool interp, double Emax, double Estep) method. This is quite an odd signature: the name implies it enables interpolation, yet it also takes an argument which must be true. Furthermore, the remaining parameters are not even used if one wishes to disable it instead.

I encountered something similar in the SphericalHarmonic class. There I decided to split this method into two: one to enable, and another to disable. I propose we do the same here.