Error checking of StencilI/StencilJ/StencilK variables

[babrodtk]

The following should give errors:

i = StencilI()
j = StencilJ()
$ u@i,j@ = 1/8 * ( 4*u_0@i,k@ ) $
$ u@i,k@ = 1/8 * ( 4*u_0@i,j@ ) $

also

i = StencilI()
k = StencilJ()
$ u@i,j@ = 1/8 * ( 4*u_0@i,k@ ) $
$ u@i,k@ = 1/8 * ( 4*u_0@i,j@ ) $