Synopsis
		sfanalytical < in.rsf > out.rsf vel=y order=3 yshot=x0 + 0.5*(nx-1)*dx zshot=z0

Parameters
		int order=3 interpolation accuracy for velocity   bool vel=y [y/n] y, input is velocity; n, slowness   float yshot=x0 + 0.5*(nx-1)*dx   float zshot=z0 read velocity or slowness