Revisited #demoted_tenancy? and added examples

Renamed #is_demoted_tenancy? to #demoted_tenancy as it sits better
with Ruby's idioms
Added #date_and_court_set? for DemotedTenancy
Using instances for conditions instead of the hash
Added #only_start_date_present? added to Tenancy

It's for the logic that will be used to populate section 4 a) & b)
Module 4 Tenancy options logic

Implemented the logic which checks Yes|No|N/A checkboxes
Tightened the tests
Fix for the failing test

The test was based on previously desired assumption that is no longer
correct.
Merge pull request #123 from ministryofjustice/67565922/re-visit-of-the-section-4-logic

Revisit of the section 4 logic