Add RefToBase member `holder_' as transient 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 saved.

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

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Add edm::reftobase::BaseHolder<*> type into a dictionary.

The following is needed for edm::RefToBase<*>, to store holder_
data member.

From Dr15Jones:
  holder_ must be stored since it holds all the information we need
  to retrieve the data. BaseHolder is an abstract base class but its
  concrete implementations ultimately hold an edm::Ref which must be
  stored

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #59 from davidlt/dict-fix-RefToBase-holder_

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