generates potential table (cubic splines) from force table by trapezoidal rule integration.
Remark: Tables should be equidistant cubic splines listed as a matrix with 5 columns: radius, potential (Y), 1.-coef., 2.-coef, 3.-coef. The definition of the spline coefficients is: E = ( ( C(J,3)*D+C(J,2) )*D+C(J,1) )*D + Y(J)