GRHD: impose atmosphere when momentum admissibility condition is violated

The condition is from arXiv:1306.4953 where they suggest to rescale the momemtum to fix this. I think that rescaling is as ambiguous as resetting to atmosphere. I decided to to the latter now as this is how I wrote it up in the thesis.

added GRHD update reftest labels

enabled an automatic merge when the pipeline for 53e59fca succeeds

merged

mentioned in commit acd803ca