PLT-427 - Periodic schedules

Add data structures and logic to generate schedules
Move Frequency and RollConvention
Add StubConvention

Data structure `PeriodicScheduleDefn` is used to define a periodic schedule. The roll convention, stub convention and explicit dates are combined with the frequency to create the schedule in a logical manner. The result is a `PeriodicSchedule` formed of one or more `SchedulePeriod`. This schedule should be capable of use by swaps and bonds, although bond schedules tend to be simpler.
This also means that for creating simple schedules either for providing example code or for writing tests, we can do it in a few lines of code. But the flexibility is available for the complex examples too.