add print if nf != 3

OCTREE/octree_interpolate_three.f90

@@ -55,6 +55,8 @@ if (y.gt.1.d0-1.d-11) yt=1.d0-1.d-11
 if (z.lt.1.e-11) zt=1.e-11
 if (z.gt.1.d0-1.d-11) zt=1.d0-1.d-11
 
+if (nf.ne.3) write(*,*) 'error: octree_interpolate_three, nf = ', nf
+
 do kkk=-1,1,2
    do jjj=-1,1,2
       do iii=-1,1,2