avoid spurious copy of MCVertex

Due to mistake in MCVertex which allows the implicit construction of an MCVertex from a pointer to an MCVertex, calling a function which accepts LHCb::MCVertex const& with a LHCb::MCVertex const* leads to the construction of a temporary MCVertex. This MR derefences the pointer when calling such a function to avoid 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)