Changed default exponential cutoff function. Function used to be exp(-8*x**3) with x=(r-r1)/(r2-r1) with a hard cutoff at x=1. This jump in energy (and force) could cause problems with energy minimization. New cutoff uses and x**3 and x**4 to correct the exponential to go to zero, zero derivative and zero curvature a x=1. Changes in energy with respect to the old formulation should be at most 0.3%, but much smaller in most cases.
Added cutoff function module. Currently contains trigonometric and exponential cutoff.
Added unittesting framework. First test for cutoff functions added. Tests whether they go from 0 to 1 and numerically test the derivative.
Added README for unittests.