Solved initialisation and pole issue in VTOSLL model.

Summary:
Solved initialisation and pole issue in VTOSLL model. Added two new test files for this model as well.

Implemented const where possible and needed as per John's comments.

Avoided unnecessary if loop and double initialization. Use static constexpr for pole size.

Test Plan:
Distributions associated with propagator q2(ll) changed due to pole compensation.
Probmax issue should not happen when running the tests.

Reviewers: tlatham, jback, kreps

Reviewed By: tlatham, jback, kreps

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D97