Some large matrices are held in levels. Clean up aggregation.py and classical.py (etc) to no retain C, AggOp, T, etc.

This is helpful: http://code.activestate.com/recipes/546530/

Some of my visualization scripts use AggOp and B from coarse levels. Maybe we should have a "kitchen sink" flag that is by default False, but when True stores all of these matrices for diagnostic purposes.

Ok, I think we should put a verbose flag in there. It's silly to keep these huge operators around, but JBS is right that we need them for diagnostics. Particularly AggOp. B will always be retained.

On the other hand, if it's only AggOp, then we can get this with just P by feeding a single B and using no prolongation smoothing

now using 'keep' to keep extra info in the hierarchy if needed

moved to github: https://github.com/pyamg/pyamg/issues