A period, i.e. a time interval suitable for securities with regular payment schedules.
For example, the accrual period is usually given by the interval periodStart, periodEnd, where as fixing date and payment date
can be adjusted versions of periodStart and periodsEnd, e.g. via fixing offsets and payment offsets.
Period implement the Comparable interface by simply using getPeriodEnd().compareTo(), i.e., the ordering is
determined by periodEnd only.
For a list of subsequent (sorted) periods it is often assumed that periodStart agrees with the periodEnd of the preceeding period,
resulting in a time-discretization.