Fix problem with tensor particle rotation to helicity basis

Summary:
Bug in EvtTensorParticle::rotateToHelicityBasis led to rotations being
compounded between generated decays

See also !33

Test Plan:
Added tests to reproduce reported bug
Fixed by implemented changes

Reviewers: abudinen, jback, kreps

Reviewed By: kreps

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D133