Change default atmosphere parameters

Traditonally, the default atmospheric model follow Linsley's parameterization of the US standard atmosphere. We know that this model leads to unphysical artifacts, e.g. in the distributions of muon production depth, due to discontinuities in density. I propose that we change the default to Keilhauer's parameterization. It has the same total grammage but does not suffer from discontinuities. It is already available in CORSIKA 8 (AtmosphereId::USStdBK).