[Users] MHD fluxes incorrectly computed in GRHydro

[Roland Haas]

Dear Einstein Toolkit user

about three months ago Hannah Klion found and fixed a bug in the
GRHydro code in the current release of the Einstein Toolkit.

You can see the commit here

https://bitbucket.org/einsteintoolkit/einsteinevolve/commits/c79140d

c79140d - Fixed bug in calculation of conserved S vector in MHD Prim2Con

This bug has since been fixed and the fix backported to the current
release branch (2018_02, "Tesla").

Who was affected by this?
Anyone who used GRHydro with a non-zero magnetic field using the C++
code has been affected. The C++ code has been the default version used
since the ET_2015_05 "Hilbert" release.

What is the effect of this bug?
The bug causes the conserved momentum density Si used to compute the
numerical fluxes to be incorrectly computed, missing a factor of the square
root of the metric determinant in front of the magnetic field
contribution. This did not affect the initial conversion from primitive
variables to conserved variables, which uses the old, correct Fortran
code. Only the flux computation was affected.

The effect will be strongest in regions where the magnetic field is
strong and in regions of strong gravitational fields.

How do I update an existing checkout?
You can use git to update to the newest version of the release or
development branches:

cd repos/einsteinevolve
git pull

On behalf of the Einstein Toolkit maintainers,
Roland Haas

-- 
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