implemented unconventional D3Q27 stencil by Abbas Fakhari

In the paper 3D phase-field is written :

As for the discrete velocity set of the D3Q27 lattice used for the hydro- dynamics, a slightly unconventional ordering was used in order to align the first 15 velocity directions with the D3Q15 model.

Therefore I also implemented this stencil with the name fakhari. For the D3Q15 stencil they used: premnath

lbmpy/stencils.py

@@ -83,6 +83,12 @@ get_stencil.data = {
                      (1, 0, 1), (-1, 0, 1), (1, 0, -1), (-1, 0, -1),
                      (0, 1, 1), (0, -1, 1), (0, 1, -1), (0, -1, -1),
                      (1, 1, 1), (-1, 1, 1), (1, -1, 1), (-1, -1, 1),
-                     (1, 1, -1), (-1, 1, -1), (1, -1, -1), (-1, -1, -1))
+                     (1, 1, -1), (-1, 1, -1), (1, -1, -1), (-1, -1, -1)),
+        'fakhari': ((0, 0, 0),
+                    (1, 0, 0), (-1, 0, 0), (0, 1, 0), (0, -1, 0), (0, 0, 1), (0, 0, -1),
+                    (1, 1, 1), (-1, 1, 1), (1, -1, 1), (-1, -1, 1),
+                    (1, 1, -1), (-1, 1, -1), (1, -1, -1), (-1, -1, -1),
+                    (1, 1, 0), (-1, 1, 0), (1, -1, 0), (-1, -1, 0),
+                    (1, 0, 1), (-1, 0, 1), (1, 0, -1), (-1, 0, -1), (0, 1, 1), (0, -1, 1), (0, 1, -1), (0, -1, -1)),
     }
 }