AthenaHitsVector : Try to add policies ala DataVector (OWN/VIEW)

AthenaHitsVector :

• What we currently have is always owning the elements (OWN). Try to add a VIEW policy ala DataVector.
• The default is OWN so nothing should change.

The reason is that in my understanding if we want to use a DataPool for the T/P converters of Hits collection we can not have the Container deleting the elements (as the DataPool owns them the container must be VIEW)

So at the end the main "goal" is to have kind of this

void Clear() {
    // delete pointers if we own the elements
    if (m_ownPolicy == OWN_ELEMENTS) {
      for (unsigned int i = 0; i < m_hitvector.size(); i++)
        delete m_hitvector[i];
    }

Similar changes happened in "resize" etc. We want to delete only if we own. And we want to deep copy only if own etc...

Ping @jchapman from simulation , @ssnyder from the expert / DataPool side , and @tadej since he brought the issue of many allocations that we might want to avoid via a DataPool