Moved all the preprocess parameter functions in aggregate.py to utils.py
and the associated unit tests to utils/test/
Changed name of Bimprove to improve_candidates
Changed the name of the preprocess_str_or_agg, preprocess_Bimprove and
preprocess_smooth to levelize_strength_or_aggregation and
levelize_strength_or_aggregation, which combines preprocess_Bimprove
and preprocess_smooth.  Improved the documention of these functions.
Moved these functions to utils.py, which is a better home for them.
Fixed a divide by zero warning from smooth.py
Missed something in the last commit to fix the divide by zero
warnings from smooth.py
Fixed typo in algebraic_distances
Merge branch 'master' of https://github.com/pyamg/pyamg