Improvements in tutorials/demos

- typos, more content
- added numba to benchmark comparison

lbmpy/session.py

 from pystencils.sympy_gmpy_bug_workaround import *
+import pystencils as ps
 import sympy as sp
 import numpy as np
 from lbmpy.scenarios import *
@@ -10,4 +11,4 @@ from lbmpy.lbstep import LatticeBoltzmannStep
 from lbmpy.geometry import *
 from lbmpy.parameterization import ScalingWidget
 import lbmpy.plot2d as plt
-from pystencils.jupytersetup import *
+from pystencils.jupytersetup import *
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