add DiffusionDirichlet boundary condition

Add a Dirichlet boundary condition for Advection-Diffusion problems.

lbmpy_tests/test_boundary_handling.py

@@ -145,8 +145,8 @@ def test_boundary_utility_functions():
 
     neumann = NeumannByCopy(name="Neumann")
     assert neumann == NeumannByCopy(name="Neumann")
-    assert not neumann == NeumannByCopy(name="Neumann")
+    assert not neumann == NeumannByCopy(name="test")
 
     stream = StreamInConstant(constant=1.0, name="stream")
     assert stream == StreamInConstant(constant=1.0, name="stream")
-    assert not stream == StreamInConstant(constant=1.0, name="stream")
+    assert not stream == StreamInConstant(constant=1.0, name="test")