TwoPunctures swap_xz results in left-handed coordinate system

[Zach Etienne]

A word of caution:

Swapping x<->z in TwoPunctures (parameter swap_xz = true) results in a non-right-handed coordinate system (coordinate permutation zyx). This flips the sign of the y-component of the BH spin vectors away from what one might expect.

In BH@H (https://github.com/nrpy/nrpy/blob/main/nrpy/infrastructures/BHaH/general_relativity/TwoPunctures/TP_interp.py) I implemented a permutation xyz → zxy in TwoPunctures that maintains right-handedness, so the BHs will orbit on the zx plane instead of the xy plane. The inputs, including y-components of spin vectors, are consistent with the BHs initially orbiting on the xy plane, which is generally assumed in PN theory.

I would recommend implementing a warning in TwoPunctures when swap_xz is enabled, to make sure the user is aware of the implications of swapping x and z.

[Roland Haas]

I don’t know. We generally assume people know what they are doing (they are doing science after all, one assumes a level of care). Outputting a warning all the time just b/c a (documented) parameter is being set, does not seem right.

If a warning is desired I would think a textual warning in param.ccl is sufficient.