slr:: formula_exp now returns 0 for vel==0 instead of nan

nonisotropic.cpp

@@ -2263,6 +2263,8 @@ EX namespace rots {
     bool sp = sphere;
     ld K = sp ? 1 : -1;
 
+    if(vel[0] == 0 && vel[1] == 0 && vel[2] == 0) return C0;
+  
     ld len = hypot_d(3, vel);
   
     if(vel[2] < 0) len = -len;