Avoid holding excessive numbers of file handles open

[Dr15Jones]

When running threaded jobs with 10 threads, the job would fail because
it ran out of file handles. This change allows the code to only open
one instance of the TFile and then closes the file once all necessary
information has been read. This works since the objects pulled from
the TFile are cloned so memory ownership is transfered.

As part of the change, memory management was simplified via the use
of std::unique_ptr.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_2_X.

Avoid holding excessive numbers of file handles open

It involves the following packages:

RecoTauTag/RecoTau

@nclopezo, @cmsbuild, @Degano, @StoyanStoynev, @slava77 can you please review it and eventually sign? Thanks.
@jpavel this is something you requested to watch as well.
You can sign-off by replying to this message having '+1' in the first line of your reply.
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[cmsbuild]

+1
Tested at: aae2b36
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-5080/718/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-5080/718/summary.html

[StoyanStoynev]

+1
Tested aae2b36 on top of CMSSW_7_2_X_2014-08-26-1400.
Following some indications in jenkins tests I ran extended tests - I found no differences in standard reco histograms (FWLite and DQM tests, in particular wf 202 and 38 for the latter) and no other issues. As expected. I did not test multi-threading.

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_2_X IBs unless changes (tests are also fine).