ODE NAN INF derivs

[Kevin Hobbs]

I'm porting a neuron model to GSL. The trouble is it has lots of logs 
and square roots, and a square wave that causes some rather rapid 
changes. Rather often a sudden change occurs that causes a derivative to 
become NAN. With the way NAN propagates and since any comparison to NAN 
is false I think the step size h will not decrease.  If I have this 
right in the section in cstd.c :

double rmax = DBL_MIN;
size_t i;

for(i=0; i<dim; i++) {
   const double D0 =
     eps_rel * (a_y * fabs(y[i]) + a_dydt * fabs(h_old * yp[i]))
       + eps_abs;
   const double r  = fabs(yerr[i]) / fabs(D0);
   rmax = GSL_MAX_DBL(r, rmax);
   }

any time there is NAN in yp[i] rmax will not change. The step will be 
accepted.

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. 
Perhapses the step size could be immediately reduced.

I already tried trapping the NANs and assigning weird things to them, 
inf, -inf, alternating + and - inf... but then all my weirdness just got 
accepted.
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