Cleanups and changes in DetDesc to streamline the code and prepare DD4hep introduction

Among other things :

• Dropped unused VPTrackingInfo
• Merged VPGeometry into the VPDetectorElement
• Improved the VP DetectorElement interface to not expose the internals
• Allow functional algorithms using conditions to have non default base class

One can expect tiny changes to the test outputs, but there should be not changes in the counters or histograms.

goes along with LHCb!2246 (merged), Lbcom!391 (merged), Alignment!107 (merged) and Boole!248 (merged)

Sits on top of !1801 (merged) and related