[RFC] Add RefToBase member `holder_'  in Reflex dicts.

RefToBase is a templated class with a private memeber `holder_` of `reftobase::BaseHolder<value_type>*` type. The following type is not part of any dictionary, thus `holder_` is not serialized.

There are two options:

1. Make `holder_` transient, which would match the current behavior (not saving it), I assume.
1. Adding a dictionary for `reftobase::BaseHolder<value_type>*` to make `holder_` serializable by ROOT.

Example error generated by ROOT:

```
Error in <TStreamerInfo::Build:>: edm::RefToBase<reco::Electron>:
edm::reftobase::BaseHolder<reco::Electron>* has no streamer or
dictionary, data member holder_ will not be saved
```

The following should resolve errors for 9 dictionaries:

- `edm::RefToBase<reco::WMuNuCandidatePtr>`
- `edm::RefToBase<reco::WMuNuCandidate>`
- `edm::RefToBase<reco::PhotonCore>`
- `edm::RefToBase<reco::Photon>`
- `edm::RefToBase<reco::Muon>`
- `edm::RefToBase<reco::GsfElectronCore>`
- `edm::RefToBase<reco::GsfElectron>`
- `edm::RefToBase<reco::Electron>`
- `edm::RefToBase<PSimHit>`

You can test using `edmClassStorageSize`, e.g.:

```
edmClassStorageSize 'edm::RefToBase<reco::GsfElectronCore>'
```

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>