Darcy-Brinkman model

What this MR does / why does DuMux need it:

Adapts the navier stokes model to allow for a Darcy Brinkman single Domain model.

Notes for the reviewer

• Velocity reconstruction at the face center allows for a full anisotropic K tensor.
• Unit test for the reconstruction methods
• Test for a darcy-brinkman obstacle in a freeflow channel. anisotropic K tensor.

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