avoid spurious implicit copy of MCVertex

Gerhard Ravenrequested to merge
fix-spurious-mcvertex-copy into master

Due to mistake in MCVertex, it is possible to assign a LHCb::MCVertex const* to a LHCb::MCVertex const& by making an implicit(!) temporary MCVertex. This MR avoids the copy.

Note: LHCb!4843 (merged) will remove this implicit conversion, and hence this MR must either be merged prior to, or together with, LHCb!4843 (merged)

Edited by Gerhard Raven

Merge request reports