added simplest TGV example

examples/00_simplest_TGV.py

@@ -0,0 +1,21 @@
+"""
+This file showcases the simplicity of the lettuce code.
+The following code will run a two-dimensional Taylor-Green vortex on GPU.
+"""
+
+import torch
+import lettuce as lt
+
+flow = lt.TaylorGreenVortex(
+    lt.Context(dtype=torch.float64),  # for running on cpu: device='cpu'
+    resolution=128,
+    reynolds_number=100,
+    mach_number=0.05,
+    stencil=lt.D2Q9
+)
+simulation = lt.Simulation(
+    flow=flow,
+    collision=lt.BGKCollision(tau=flow.units.relaxation_parameter_lu),
+    reporter=[])
+mlups = simulation(num_steps=1000)
+print("Performance in MLUPS:", mlups)