refactor SusySelection::setSysts

Details:
break it down into
- toggleNominal
- toggleStdSystematics
- toggleFakeSystematics
Add std systematic list, and initialize the fake syst histos when needed.
Validated
add HistFitterTree
simple regression tests
move passThirdLeptonVeto from SusySelection to susy::wh::kin
SusyPlotter: add a few syst variations, add --with-syst opt
skeleton HtfFiller
add first version of susy::wh::Systematics and their utils
Systematics: add unit test fix and reshuffle things around
SusySelection::passSrSs(): drop obsolete WH_SR argument
drop obsolete WH_SR enum
drop useless passSrSsBase
drop obsolete passSR[6789]*
EventFlags: initial version
SsPassFlags: add ee,em,mm; implement updateLlFlags()
refactor selectEvent ad passSrSs

Details:
Refactor SusySelection::selectEvent and SusySelection::passSrSs,
splitting each of these functions in two logical steps:
- determine the flags
- increment the counters
This split is necessary when looping over systematics, so that
we don't increment N times the same counters.
This is a large commit, so here are the technical details:
- split selectEvent in computeEventFlags, incrementCounters
- split passSrSs in computeSsFlags, incrementCounters
- add the struct EventFlags

Tested with SusyPlotter that the cutflow is unchanged (but
modified other classes as well).
ntsys2sys: comment ill-defined cases, so that the compiler reminds me of this
drop channel-specific control regions (validated, see Feb_02 control plots)
SusyPlotter: drop unused isEe/isMm/isOf vars
move increment(n_pass_category) to increment counters (avoid multiple increment with syst)
drop n_pass_muIso, n_pass_elD0Sig counters (now done in SusyNt)