Add package for primary numbers of passive materials

Hi,

(please do not squash commits for this MR, since the separate commits document different fixes)

this MR adds a package hosting the primary numbers of the passive material description of the muon spectrometer and its surrounding. The package also has a README file which will be used to document which xml file will be used in which ATLAS detector layout. Since this is the initial MR for this new package, only the Run2 geometry file is added. The others will follow in a follow-up MR.

Best, Nico