Fixed bug with 5th input parameters of BTOXSLL model

Summary:
Fix bug whereby specifying 5th parameter of BTOXSLL model would cause first 4 to be ignored.

Added tests for the bug - required small modifications to json file checker.

This is associated with !44 and T274.

Test Plan: New tests for Belle (4 arguments) and Belle II (5 arguments) give identical results.

Reviewers: kreps, tlatham, jback

Reviewed By: tlatham

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D136