Add SettleToConstantQuaternion

[geoffrey4444]

Proposed changes

Add new function of time like SettleToConstant but for quaternions; the function of time always returned a unit quaternion (i.e., a rotation).

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Further comments

[knelli2]

Looks good. Please also fix the clang-tidy and compilation errors as well.

[knelli2]

Looks good, you can squash. (you can optionally wait for #5706 to be merged and rebase so the tests pass)

[geoffrey4444]

Thanks again for the review, @knelli2 ! I squashed. A GrMhd test failure on gcc-9 release is unrelated to this PR (looks like tolerances might be too strict?)

[knelli2]

Test failures unrealted