MINIAOD: reducing space taken by PV covariance - 74X version

[arizzi]

for vertices beyond pv[0] the covariance information is reduced to an 11bits mantissa (instead of 23).
This reduces the footprint of PVs by about 200bytes/event.

(same as #8157 but targetting 74X)
@sethzenz @VinInn @gpetruc

[cmsbuild]

A new Pull Request was created by @arizzi for CMSSW_7_4_X.

MINIAOD: reducing space taken by PV covariance - 74X version

It involves the following packages:

DataFormats/PatCandidates
PhysicsTools/PatAlgos

@cmsbuild, @vadler, @nclopezo, @monttj can you please review it and eventually sign? Thanks.
@rappoccio, @imarches, @ahinzmann, @acaudron, @mmarionncern, @jdolen, @nhanvtran, @schoef, @ferencek, @mariadalfonso, @pvmulder, @TaiSakuma this is something you requested to watch as well.
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[cmsbuild]

+1
Tested at: 5bcf932
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8158/3245/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8158/3245/summary.html