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GRHD: Add option to disable the cartoon derivatives when running with the cartoon formulation

Florian Atteneder requested to merge fa/disable-doublecartoon into main

Adds the following option to the GRHD section:

  • doublecartoon_disable_derivatives: If true, then the doublecartoon derivative terms are not added to the source terms.

It appears that this option is senseless, because why even use the doublecartoon formulation in the first place. However, it turns out that these derivative terms severely degrade the quality of any TOV star tests I tried so far, and removing those terms from the equation helps a ton. The reason is likely that these terms contain 1/r (or 1/x) factors which will amplify any noise coming from the TOV star's surface and propagating inwards.

Note that removing those terms changes the PDEs we solve. In fact, removing these terms was already considered in Bugner's paper Physical Review D, 2016.

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