LHCbMath: add summation operators for WStatEntity (@ibelyaev)

Cherry picked from !1385 (merged) on run2-patches

• [2018-07-02 00:14] Validation started with lhcb-head#1905
• [2018-07-02 00:14] Validation started with lhcb-lcg-dev3#573
• [2018-07-02 00:16] Validation started with lhcb-tdr-test#201
• [2018-07-02 00:16] Validation started with lhcb-lcg-dev4#573
• [2018-07-02 00:17] Validation started with lhcb-gaudi-head#1918

merged

mentioned in commit bc9c61af