First example of storing in tree branches objects instead of arrays of
integral types

This commit starts a migration of representation of physics objects in
trees to dedicated C++ classes. Eventually, it will allow to develop a
compact representation and resolve issue #29.

The impelementation follows the documentation in [1] and examples in
DataFormats/PatCandidates.

[1] https://twiki.cern.ch/twiki/bin/view/CMSPublic/SWGuideCreatingNewProducts
A draft conversion of electrons to the new class-based storage

Kept old plain branches for a comparison.
Remove plain branches for electrons

This finalises the transition for class-based storage for electrons. I
have checked that the information stored is same as before.
Convert muons to class-based storage

Old branches are kept to allow the comparison.
Finalise conversion to the class-based storage for muons

Remove plain branches. I have verified that the new way of storage leads
to same results.
Convert MET to the class-based storage

I have verified that stored data are same as used to be before with
plain branches.
Perform a draft conversion of jets to the class-based storage

Old plain branches are kept to faciliate comparison.
Finalise the migration of jets to the class-based storage

Most likely, will incorporate the generator-level information on jet
(e.g., true flavours) into the same class, but for the time being keep
it in independent plain branches.
Move jet flavours to the new Jet class

In case of real data the corresponding data members will be set to zero
and will be compressed efficienly.
Move information about a matching to a GEN jet to the new Jet class

This finalises the transition of jets to the new class-based storage.
Migrate pile-up info to the class-based storage

Drop the number of pile-up interactions in each bunch crossing: it has
never been used so far, and it will occupy quite some space in
run-dependent MC. Store only in-time pile-up instead.
Migrate basic generator inforamation to the class-based storage

In the new class save energy scales in PDF and ME independently.
Remove ME energy scale from generator information

It is always same as the PDF scale in the relval ttbar, and it should be
true for all samples.
Move saving of particles from the hard interaction to a dedicated plugin

The new plugin is a draft. Will need to convert it to class-base
storage.
Migrate event ID to the class-based storage

Unlike all other objects, the migration of event ID cannot be exploited
to compress data in a more efficient way. Instead, it is done for a
uniformity of treatment.
Merge trees in the PlainEventContent plugin

After the convertion to the class-based storage, there is no need to
split information among several trees.
Update documentation for the PlainEventContent plugin
Convert the plugin HardInteractionInfo for the class-based storage
Add Reset method to all data formats which still lack it

It is important when same object is recycled many times, e.g., to fill a
vector. This is a typical use case.
Migrate the GenJetInfo plugin to the class-based storage