[ET Trac] #2889: Outflow uses incorrect `sf_centroid` to compute surface coordinates

[Roland Haas]

#2889: Outflow uses incorrect `sf_centroid` to compute surface coordinates

 Reporter: Roland Haas
   Status: new
Milestone: 
  Version: 
     Type: bug
 Priority: major
Component: EinsteinToolkit thorn

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_radius`if 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
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