Refactoring in Event.cc (#122); updated associated tests

• Removed asserts for <2 bosons.
• Removes the classification of >1 boson events as bad_final_state, and instead are non_resummable via implemented_types().
• final_state_ok checks for an expected number of decays given the

boson content of the event; this may need revisiting if stable-W is
desirable.

• Remove 2-boson tests from test_classify
• Modify comment in test_decay to reflect the test intent for an unexpected decay