Rearranged short_range code in msm.c so k loop on outside and conditionals
moved outside of loops to the end of the method.
Moved "bin to bin" logic into self-/neighbor- interaction methods, even
powers only does assert if more than 0 items are expected, even powers now
has spline evaluation as "most common case" for if-clause. dynmem only
asserting that returned pointer is not NULL if size > 0.