Vector direction

[mlund]

If I get it correctly, with the current definition of r, the function returns the force acting on charge "b". Perhaps we could change to @brief force on charge a due to charge b? Due to a lucky cancellation of signs, we got the right force in Faunus, but it's quite easy to make mistakes. I renamed distance vectors to a_towards_b to make it very explicit what is expected.

coulombgalore/coulombgalore.h

Line 1018 in d0c5862

* @param r distance-vector between charges, @f$ {\bf r} = {\bf r}_{z_B} - {\bf r}_{z_A} @f$, UNIT: [ input length ]