Fix Python interface compilation

* ensure -fno-rtti is used
* remove warnings
First sketch of a more pythonic interface to isl

* add a few operators
* use function predicate to define sets and maps
Support paused iteration over a set

through threading...
Draft document for the tree extension
Add an OO section

Additionally, fix a few typos.
Update schedule_tree spec to reflect recent discussions

* Add the gemver code transformation
* Add a note concerning the canonic tree