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