1. Started reorganizing the SeparatrixCollections classes such that
there is a SepSegmentCollection type, and there are versions of the
SepSegmentCollections class that can iterate over SepSegmentCollection
in different ways.

2. Added the Mod1NumberIntExchange class that keeps track of numbers as
linear conbinations of the lengths of an interval exchange.

3. Moved the enums LEFT, RIGHT, DOWN, UP under a unified Direction
namespace.

4. These changes affect most of the code so a lot of changes has to be
adjusted and that hasn't finished yet.