Outflow incorrectly used sf_centroid as the origin point for the spherical surface it computes fluxes on. However for SphericalSurface the surface is defined with respect to sf_origin via sf_radius[ind2d] .

This pull request fixes this and also introduces a runtime parameter to chose between sf_centroid and sf_origin. The former is useful if one also sets override_radiusif eg the sphericalsurface is really just a tracker position.

Thankfully for the common case where there are either a set of spheres centered on on the coordinate origin or spheres following a neutron star via tracking we usually have sf_origin and sf_centroid being identical. However flow through an apparent horizon would be incorrect. I would suspect that sf_origin is the centroid of the previous horizon find result.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2889/outflow-uses-incorrect-sf_centroid-to