ODE NAN INF derivs

[Brian Gough]

Kevin Hobbs writes:
 > I'd like advice on adding a way for my derivative function to reject the 
 > step, say by returning GSL_ERANGE, without terminating the model. 

Your function(s) should return an error code instead of GSL_SUCCESS if
you want to avoid proceeding with NaN values.  The library should
propagate the error code back up to the calling level, so you can
retry with a different step-size.  If it doesn't, that is a bug.

-- 
Brian Gough