LoKiCore Kinematics.h/.cpp: fixes to `deltaM2` (@ibelyaev)

Marco Cattaneorequested to merge
cherry-pick-ea0742c7 into master

  • minor fix in deltaM2 function (better treatment of low-momenta)

  • add the variant of deltaM2 function with the forced assignment of the same massLoKiCore Kinematics.h/.cpp

Cherry picked from !1627 (merged) on run2-patches

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