Slip Bounce-Back Boundary Condition

[PhiSpel]

This boundary condition could help with emulating symmetrical flows, e.g., TGV or Couette flow (when initialized without pertubations).

Good example for Ben to also include a test.

• [ ] Implement slip bb-bc in boundaries.py - e.g. test_slip_bb_boundary_mirrored_couette() to compare analytic solutions
• [ ] Implement example in examples/simple_flows/FlatPlateSlip and examples/simple_flows/halfed_couette.py
• [ ] Implement test in tests/test_boundaries.py