Switch to using VectorDoubleAttribute for GenVertex weights for consistency with internal HepMC3 code
The HepMC3 WriterAsciiHepMC2 assumes VectorDoubleAttribute has been used for the GenVertex "weights" property. As this class is used in a couple of places in the Athena code. We have to follow this convention in our transient GenVertex EDM, even though the information is persistified as std::vector<float>.
As there don't seem to be any clients for this information after an initial inspection, it may be that we can drop this information from the transient EDM in the future. We need to confirm that there are no clients first though.
Replaces !59134 (closed).