Synopsis
		sfduffing < in.rsf > out.rsf sfile=sfile.rsf gamma=0.75 omega=1 kxi=1 x0=0 y0=0 pow1=1 pow2=3 verb=n ricker=n

Duffing equation: x'' + 0.5 x' - x + x^3 = gamma cos(omega t) + kxi input(t)

Parameters
		float gamma=0.75 strength of external force   float kxi=1 adjustment for input signal   float omega=1 angular frequence of external force   int pow1=1 power of first non-linear restitution term   int pow2=3 power of second non-linear restitution term   bool ricker=n [y/n] if y need extenal input for external force   string sfile= auxiliary input file name   bool verb=n [y/n] verbosity flag   float x0=0 initial value of x0   float y0=0 initial value of y0