Fix FreeSlip boundary condition

There is an issue with the proxy field substitution and the FreeSlip boundary condition. This is true in particular if the normal vector is not pre-determined as implemented in walberla/walberla!520 (merged).

Currently, with the pull pattern the following substitutions are performed:

f_in[0,0](invdir[dir]) ← f_in[{indexField}_{0}^{wnx},{indexField}_{0}^{wny}](indexField[0](ref_dir))
->
pdfs[{f_{in inv offsets x}}_{dir},{f_{in inv offsets y}}_{dir},{f_{in inv offsets z}}_{dir}](f_in_inv_dir_idx[dir]) ← pdfs[{indexVector}_{0}^{wnx} + {f_{in offsets x}}_{{indexVector}_{0}^{ref_dir}},{indexVector}_{0}^{wny} + {f_{in offsets y}}_{{indexVector}_{0}^{ref_dir}},{indexVector}_{0}^{wnz} + {f_{in offsets z}}_{{indexVector}_{0}^{ref_dir}}](indexVector[0](ref_dir))

Whereas the correct substitutions are:

f_in[0,0](invdir[dir]) ← f_in[{indexField}_{0}^{wnx},{indexField}_{0}^{wny}](indexField[0](ref_dir))
->
pdfs[{f_{in inv offsets x}}_{dir},{f_{in inv offsets y}}_{dir},{f_{in inv offsets z}}_{dir}](f_in_inv_dir_idx[dir]) ← pdfs[{indexVector}_{0}^{wnx} + {f_{in inv offsets x}}_{dir},{indexVector}_{0}^{wny} + {f_{in inv offsets y}}_{dir},{indexVector}_{0}^{wnz} + {f_{in inv offsets z}}_{dir}](indexVector[0](ref_dir))

The problem is that the population index (ref_dir) is used to determine a spatial offset. This MR in its current form fixes this issue for the particular case of the FreeSlip condition and the pull pattern. Other configurations break.

lbmpy/advanced_streaming/indexing.py

@@ -88,10 +88,10 @@ class BetweenTimestepsIndexing:
 
         if (f_dir, inverse) in self._trivial_offset_translations:
             offsets = (0, ) * self._dim
-        elif f_dir == 'in':
-            offset_array_symbols = self._offset_array_symbols(f_dir, True)
-            offsets = tuple(sp.IndexedBase(s, shape=(1,))[self.dir_symbol] for s in offset_array_symbols)
-            self._required_offset_arrays.add((f_dir, True))
+        else:
+            offset_array_symbols = self._offset_array_symbols(f_dir, inverse)
+            offsets = tuple(sp.IndexedBase(s, shape=(1,))[index] for s in offset_array_symbols)
+            self._required_offset_arrays.add((f_dir, inverse))
 
         return {'index': translated_index, 'offsets': offsets}