LHCbMath:add summation operators for WStatEntity

LHCbMath:

• add summation operators for WStatEntity. They are very useful for ROOT/(T,R)DataFrame parallelization actions

This MR for run2-patches branch, it will be great to propagate it also for master branch,

added 1 commit

• 29102cb8 - use default constructors

Compare with previous version

• [2018-06-24 00:04] Validation started with lhcb-run2-patches#55
• [2018-06-25 00:08] Validation started with lhcb-run2-patches#56
• [2018-06-26 00:07] Validation started with lhcb-run2-patches#57
• [2018-06-27 00:08] Validation started with lhcb-run2-patches#58
• [2018-06-28 00:06] Validation started with lhcb-run2-patches#59
• [2018-06-29 00:08] Validation started with lhcb-run2-patches#60
• [2018-06-30 00:14] Validation started with lhcb-run2-patches#61
• [2018-07-01 00:05] Validation started with lhcb-run2-patches#62

merged

mentioned in commit 6c28ea97

mentioned in commit e7adaea2

mentioned in merge request !1391 (merged)