Specialized and customized intervals

Added

• An AbstractDiscreteInterval class to ease the creation of specialized discrete intervals (experimental).
• A create_api function to generate an API similar to the one of portion but configured to use a given subclass of Interval (experimental, see Specialize & customize intervals).

Changed

• Speed up repr and to_string for Interval instances (see #76, adm271828).
• Some internal changes to ease subclassing:
  • from_string and from_data accepts a klass parameter to specify which class should be used to create Interval instances (default is Interval).
  • Add a klass parameter for open, closed, openclosed, closedopen, singleton and empty (default is Interval).
  • Add a _klass class attribute in IntervalDict to specify how to create Interval instances (default is Interval).
  • IntervalDict uses self.__class__ to preserve subclasses when creating new instances.