trav: A fortran program calculating travel time and ray parameter

The original trav programme was one component of the fk program (by Prof. Lupei Zhu). However, it encounter floatting point exception if compile as conventional use. Here I make it independent to avoid the problem and also make it more easier to use.

Here I prepared a modified ray tracing tutorial for 1-D layered velocity scenarios. The tutorial including two parts: A description file and the code file (Fortran). Download here