TrackIsolationTool -- Avoid crash if the muon does not have a primaryTrackParticle

Hi everybody,

the crash reported in ATLASG-1607 is super bizarre:

Iso...TrackIsolationToolNonprompt_All_MaxWeight500 WARNING  No TrackParticle found for muon  with pT: 12.814[GeV], eta: 2.21978 phi: 2.04518 q: -1 primaryAuthor: 8 allAuthors: 256

The muon is only reconstructed by the CaloTag algorithm and does not have a valid ElementLink to its primary track particle, which is in this case the ID-track only. However, this link had to exist sometime during the reconstruction job, because the muon has a sensible momentum which only happens, if the link is valid (cf. here). Otherwise the muon is sent to the trash-bin 😕. Anyway, the reference track particle in the TrackIsolation is used to determine the position of the eta--phi cone collecting all tracks contaminating the isolation. Given that, the values are the same for xAOD::Muon and xAOD::TrackParticle, we can just return the muon pointer itself and avoid the job crash. I try to find out in the meantime why the element link is reset during the reconstruction.

Adding also the alsoTargeting:22.0-mc20 label. @pberta if you disagree that the patch should be also swept into the reprocessing branch, then feel free to remove the label.

Tagging: @gabarone, @mvanadia, @jcatmore

Addresses: ATLASG-1607