Seeding layer cache

a few changes related to seeding layers:
- switch from cache based on DetLayers to cache based on SeedingLayers
- fix small bug in HitExtractorSTRP
- add possibility to set minAbsZ for TOB hits
- add possibility to have multiple SeedingLayers per DetLayer

tracking results expected to be unchanged (functionalities to be used in the future by strip triplet seeding)