odeiv2 request

[Patrick Alken]

I had a problem recently where I needed to integrate forward along a 
curve (positive step size h) and then integrate backward again (negative 
h). But there is currently no way to change the sign of the step size h 
using the driver routines, so I had to modify driver->h directly.

I propose adding a routine gsl_odeiv2_driver_set_h() to allow the user 
to reset the step size to what they want. Tuomo: do you see any negative 
issues with this? I can make the addition if you want.

Thanks,
Patrick