Interpretation of gradients

[martijnende]

Hi Keurfon,

Thanks for your efforts of releasing/maintaining your code projects. fteikpy has been very useful to me so far.

From the documentation and the tests it is not entirely clear to me what the tt.gradient vector represents. Are these the gradients of the travel times with respect to the source location, for each receiver? If yes, that would be convenient for gradient-based hypocentre inversion :)