Ray tracing different arguments

This is a new way of organising ray_tracing arguments, following the suggestion of Alessandro (see his py_ray_tracing branch): 

1. new wrapping function is created, ray_tracing that takes in Calibraiton *cal and returns X[3] and a[3] double arrays instead of Xb,Yb,Zb and a,b,c, respectively
2. the wrapping function calls to the old function using the previous construction