射线跟踪算法主函数

 A function to raytrace through a sound speed profile to a given time. Raytrace will trace the path sound will travel through a medium with an  isotropic horizontal sound speed profile and a vertical sound speed  profile given by (zz,cc) from a point (xo,zo) and departing at an angle  theta0 for a time of tt seconds. The path is returned in vectors x and z.  The actual time of travel is returned in t, which may be slightly greater  that tt. The sound speed profile should include a measurement at the  surface (zz = 0). If it does not, the profile will be extended to the  surface by replicating the first measurement there.  In addition to the above, raytrace() handles caustics (when rays bend  through horizontal) and reflections off the surface (z=0) and bottom  which is taken to be at the maximum depth measurement in the soun