Started implementing the creation of tt maps.

Unfortunately it is not going to work in this way. The _trace_back
method was added and some changes to the from_separatrix method. They
are committed in case they happen to be useful later, although it is
probably garbage.
Revert "Started implementing the creation of tt maps."

This reverts commit 5ff4a74e48e0d51f29a99777fb9a69df88fd2223.

Mainly messed around with the Separatrix class. Other than storing the
list of traversed intervals and transforming later into a train track
path, now it is automatically stored as a train track path (which
includes the traversed intervals twice already).
Fixing a ton of bugs.

Rewriting the Separatrix class broke the code at several other places,
not they seems mostly corrected. At least the double covers work correctly.