Implemented an improved version of the communication for periodic BC

The communication of the ghost layers used to communicate just all
values in between one time step to make sure that everything is correct.
Furthermore the communication was only valid for pull stream steps.
The improved communication distinguishes automatically between pull and
push and communicates only values which are needed. With this improvement
it was possible to implement the EsoTwist streaming scheme.

lbmpy/fieldaccess.py

@@ -5,6 +5,7 @@ from pystencils import Field
 
 
 __all__ = ['PdfFieldAccessor', 'CollideOnlyInplaceAccessor', 'StreamPullTwoFieldsAccessor',
+           'StreamPushTwoFieldsAccessor',
            'AAEvenTimeStepAccessor', 'AAOddTimeStepAccessor',
            'EsoTwistEvenTimeStepAccessor', 'EsoTwistOddTimeStepAccessor',
            'visualize_pdf_field_accessor', 'visualize_field_mapping']
@@ -58,6 +59,16 @@ class StreamPullTwoFieldsAccessor(PdfFieldAccessor):
         return [field(i) for i in range(len(stencil))]
 
 
+class StreamPushTwoFieldsAccessor(PdfFieldAccessor):
+    @staticmethod
+    def read(field, stencil):
+        return [field(i) for i in range(len(stencil))]
+
+    @staticmethod
+    def write(field, stencil):
+        return [field[d](i) for i, d in enumerate(stencil)]
+
+
 class PeriodicTwoFieldsAccessor(PdfFieldAccessor):
     """Access scheme that builds periodicity into the kernel.
 

lbmpy/lbstep.py

@@ -120,6 +120,7 @@ class LatticeBoltzmannStep:
 
         # -- Boundary Handling  & Synchronization ---
         stencil_name = method_parameters['stencil']
+
         self._sync_src = data_handling.synchronization_function([self._pdf_arr_name], stencil_name, target,
                                                                 stencil_restricted=True)
         self._sync_tmp = data_handling.synchronization_function([self._tmp_arr_name], stencil_name, target,