Synopsis
		sfztrace < in.rsf nt=nx*nz na=362 da=0.5 a0=-90. vel=y order=3 iorder=4

Parameters
		float a0=-90. starting angle (in degrees)   float da=0.5 angle increment (in degrees)   int iorder=4 interpolation accuracy for grid   int na=362 number of angles   int nt=nx*nz ray length bound   int order=3 interpolation accuracy for velocity   bool vel=y [y/n] y, input is velocity; n, slowness