WIP: Graph datahandling

This is the draft for a data handling that (optionally) forwards all calls to SerialDatahandling. All calls and data transfers get recorded for the creation of an execution graph.

Needs to be changed after the breaking changes in datahandling.

Needs a tiny change in lbmpy:

Instead of using TimeLoop(...) for time loop creation a custom function is used.

pystencils_tests/test_graph_datahandling.py

@@ -10,6 +10,8 @@
 
 import pytest
 
+pytest.importorskip('lbmpy')
+
 from lbmpy.boundaries import UBB, FixedDensity, NoSlip
 from lbmpy.geometry import add_pipe_inflow_boundary, add_pipe_walls
 from lbmpy.lbstep import LatticeBoltzmannStep
@@ -17,7 +19,6 @@ from pystencils.datahandling import create_data_handling
 from pystencils.datahandling.graph_datahandling import GraphDataHandling
 from pystencils.slicing import slice_from_direction
 
-pytest.importorskip('lbmpy')
 
 
 def create_lid_driven_cavity(domain_size=None, lid_velocity=0.005, lbm_kernel=None,