HepMC3 migration: Use auto more often when it is apropriate in the current codes

In this MR the usage of auto is expanded. Most cases are related to expressions like

auto vert=particle->end_vertex();
if (vert) {
...
}

For these cases:

• The type of the variable is clear for humans, so the readability does not suffer. It is HepMC::GenVertex* for HepMC or HepMC3::GenVertexPtr HepMC3. Modulo specifier.

• The compiler will take care of the correct specifier and avoid casts.

• The code reads the same for HepMC and HepMC3.

In addition to the auto, the HepMC::FourVector is used explicitly.

Andrii