PLT-427 - Link day count to schedules

Allow day count to query schedule
Fix '30E/360 ISDA'
Fix '30U/360'

A few day counts need more information than just two dates. Provide this information via an optional interface, and fixup day count implementations to match.

Implented design notes:
* Splitting `DayCount` into two classes has been considered and is not considered to be a useful approach.
* Forcing provision of schedule info would be overkill for most cases.

is -> if