LoKiGen: add more symbols : GMINTREE, GMAXTREE, GSUMTREE & GMULTTREE

This is a variant of MR !1375 (merged) and MR !1377 (merged) but for 2017-patches branch

Phys/LoKiGen: add new symbols

• GMINTREE : e.g. "GMINTREE( GPT , 'mu+' == GABSID) > xxx * GeV" - it can mimic certains selection/tripping criteria and also L0Muon trigger
• GMAXTREE
• GSUMTREE : e.g. "GMINTREE( GPT , ( 'pi+' == GABSID ) | ( 'K+' == GABSID ) ) > xxx * GeV" - it mimics some popular stripping/Hlt2 cuts used by Charm and BnoC groups
• GMULTTREE : "GMULTTREE( GPT , ( 'mu+' == GABSID ) ) > xxx * GeV * GeV " - it mimics L0DiMuon-TOS for eg. B-> ( J/psi -> mu mu ) X decays

They are useful to extend and simplify the syntax of LoKi::GenCutTool allowing for mimic some trigger (e.g. L0Muon and/or L0DiMuon) and/or stripping selection criteria (like scalar sum of Pt, massively used by Charm and BnoC working groups). It will allow to mimic certain selection/stripping/trigger criteria as tight generator-level cuts, drastically improving the overall performance of MC-production system.