PLT-372 - Make extensible enum implementations less fragile

Constants in interface is fragile
Adding a toString() to subclasses caused the system to break
PLT-372 - Rename day count constants

Follow existing OpenGamma usage
PLT-407 - Add date adjuster

Simplifies use of TemporalAdjuster
PLT-406 - Add roll conventions

Influenced by FpML
PLT-372 - Rename holiday calendar

HolidayCalendar makes more sense wrt the embedded constants
PLT-372 - Enhance holiday calendar

Add hasNoHolidays()
Fix bug when no holidays
Better toString()
PLT-407 - Add business day adjustment

Combines business day convention and calendar
PLT-407 - Add period adjustment

Combines a number of days and holiday calendar
Optional final business day adjustment
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