pytest and coverage improvements

- notebooks are considered in coverage analysis now
   -> custom notebook runner plugin adapted
- added images for documentation

session.py

 import sympy as sp
 import numpy as np
+from pystencils.jupytersetup import *
 from lbmpy.scenarios import *
 from lbmpy.creationfunctions import *
 from pystencils import makeSlice, showCode
@@ -7,13 +8,5 @@ from lbmpy.boundaries import *
 from lbmpy.postprocessing import *
 from lbmpy.lbstep import LatticeBoltzmannStep
 from lbmpy.geometry import *
-
-try:
-    from IPython import get_ipython
-    ipython = get_ipython()
-
-    from pystencils.jupytersetup import *
-    from lbmpy.parameterization import ScalingWidget
-    import lbmpy.plot2d as plt
-except ImportError:
-    pass
+from lbmpy.parameterization import ScalingWidget
+import lbmpy.plot2d as plt