fixed derivatives in CoordinationNumbers (#471)

I verified it, I will then modified a regtest so to keep checking for this

src/multicolvar/CoordinationNumbers.cpp

@@ -170,8 +170,8 @@ double CoordinationNumbers::compute( const unsigned& tindex, AtomValuePack& myat
       if(r_power > 0) {
         d = sqrt(d2); raised = pow( d, r_power - 1 );
         accumulateSymmetryFunction( 1, i, sw * raised * d,
-                                    (dfunc * d * raised + sw * r_power) * distance,
-                                    (-dfunc * d * raised - sw * r_power) * Tensor(distance, distance),
+                                    (dfunc * d * raised + sw * r_power * raised / d) * distance,
+                                    (-dfunc * d * raised - sw * r_power * raised / d) * Tensor(distance, distance),
                                     myatoms );
       } else {
         accumulateSymmetryFunction( 1, i, sw, (dfunc)*distance, (-dfunc)*Tensor(distance,distance), myatoms );