Improve speed of CylSplinePotential

[adrn]

Right now it is very slow because it must construct a spline object with the input grids each time the energy/gradient/density functions are called. It might be possible to store this object on the Wrapper class and pass a pointer in to C. To do this, we need to add functionality to the potential classes (actually the struct types) to support having an array of pointers that get passed around.