GRHD: impose atmosphere when momentum admissibility condition is violated...

GRHD: impose atmosphere when momentum admissibility condition is violated (!224)

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.