convert_HydroBase_to_IllinoisGRMHD.c

1. HydroBase Aphi is directly copied into IllinoisGRMHD phitilde on line 29. A comment in the interface says "# phitilde (=Phi*psi^6)", and another "sqrt{gamma} Phi", so I believe the direct copy is missing a factor of Psi^6 or sqrt(det gamma). It is also missing a mag_factor scaling of 1/4pi when compared to the Avec copying, I am not sure if it is correct without it or it should have it given mag_factor/rescale_magnetics is on by default.

2. IllinoisGRMHD Ax/Ay/Az are face centered staggered, Aphi is vertex centered. They are populated directly from HydroBase cell centered quantities. This is a bit subtle for Initial Data thorns to interface with properly, it is somewhat documented but could be a bit clearer.

convert_IllinoisGRMHD_to_HydroBase.c

1. Only HydroBase Bvec is repopulated. HydroBase Avec and Aphi are not refilled (probably due to centering mismatch). This is probably fine as the magnetic field is the more interesting quantity for diagnostics, but it could be useful for debugging or consistency.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2952/grhaylet-illinoisgrmhd-convert_-_to_-c