Package cleanup and contrib #245

This is an offline discussion that aims on simplifying Cortex packages by:

A) Reassessing whether we should break IECore into sub-packages ( IECoreMath, IECoreRendering, IECoreData, IECoreAlgo, etc)

B) Moving classes to Contrib and making them optional: old stuff, optional stuff, not so well thought out stuff. Maybe we could be more flexible in terms of binary compatibility for things moved to Contrib?

C) Use pyilmbase for Imath python bindings. What do we do with Imath extensions we have?

We can discuss here about these three big changes that may be addressed separately, once we have a plan.