XCTS: add Robin outer boundary conditions

[nilsvu]

Proposed changes

These allow to place the outer boundary at a much smaller outer radius R, since the error falls of as 1/R^2 rather than 1/R.

Here's a plot showing the faster convergence for BBH initial data:

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Further comments

[nilsdeppe]

The clang-tidy errors look legitimate. You can squash immediately

[nilsdeppe]

Does the newline break rendering? I don't remember if we needed something special for snippet

[nilsdeppe]

Move this and the boundary_condition_types to the cppto reduce hpp code? I doubt these are performance-critical to the point where the ability for the compiler to inline them helps