Stateful helper used during the calendrical merge process.
Instances of this class are used internally when merging a field-value map into a calendrical. The class is public as it is possible for additional date/time fields to be created and used by subclassing {@link DateTimeFieldRule}.
The merger is a view of the underlying calendrical. Changes to this instance affect the calendrical which created it.
Calendrical.Merger is mutable and not thread-safe. It must only be used from a single thread and must not be passed between threads.
@author Michael Nascimento Santos
@author Stephen Colebourne