add DiffusionDirichlet boundary condition

Add a Dirichlet boundary condition for Advection-Diffusion problems.

lbmpy/boundaries/__init__.py

 from lbmpy.boundaries.boundaryconditions import (
-    UBB, FixedDensity, SimpleExtrapolationOutflow, ExtrapolationOutflow, NeumannByCopy, NoSlip, StreamInConstant)
+    UBB, FixedDensity, DiffusionDirichlet, SimpleExtrapolationOutflow,
+    ExtrapolationOutflow, NeumannByCopy, NoSlip, StreamInConstant)
 from lbmpy.boundaries.boundaryhandling import LatticeBoltzmannBoundaryHandling
 
-__all__ = ['NoSlip', 'UBB', 'SimpleExtrapolationOutflow', 'ExtrapolationOutflow', 'FixedDensity', 'NeumannByCopy',
+__all__ = ['NoSlip', 'UBB', 'SimpleExtrapolationOutflow', 'ExtrapolationOutflow',
+           'FixedDensity', 'DiffusionDirichlet', 'NeumannByCopy',
            'LatticeBoltzmannBoundaryHandling', 'StreamInConstant']