Renamed pfJetsEI to ak4PFJetsCHSEI

Renamed `pfJetsEI` to `ak4PFJetsCHSEI` to make the collection name a bit more descriptive.

@deguio `pfJetsEI` also appears in https://github.com/cms-sw/cmssw/blob/CMSSW_7_4_0_pre1/DQMServices/Components/test/FrameworkJobReport_9_a_2.xml but I wasn't sure what this file is used for and whether I should update it since `runTheMatrix.py -s --useInput all` had no failed tests
For example, this is one place where having `ak4` in the collection name makes it clear what `algo` and `rParam` need to be set to in `addJetCollection(...)` .