Fix boundary condition edge diagonals

When multiple boundary conditions are applied to a single node, which subgrid distances ("q's") belong to which node must be determined.

We decided to add "diagonal" q's to the first boundary condition that is added to the node:

In this context, adding means calling the corresponding function in the GridGenerator like LevelGridBuilder::setNoSlipBoundaryCondition().

The resulting behavior should now be the same as in the CPU version. This is necessary for #54.

In some earlier versions boundary conditions could overwrite each over (added in commit a66489e4: Fix outlet issue, 09.08.2022). This is no longer the case, so some apps may need to be changed accordingly.

Tasks:

• confirm that this is exactly the same behavior as in the CPU version.
• code review
• run regression test(s)
• run test cases by Henrik and Henry

This merge is related to #66 (closed).