Ignore differences in dh/dt for the active pair test if the norm of the value...

Ignore differences in dh/dt for the active pair test if the norm of the value is less than 3e-5. This is close enough to zero.

tests/tolerance_pair_force_active.dat

 #   ID  wcount   h_dt
     0	   1      2.4e-3
     0	   1      2.4e-3
-    0	   1      1e-5
+    0	   1      3e-5