Use a numpy rather than pandas object for internal reweighting data.
Use a numpy rather than pandas object for internal reweighting data.

This is accessed a lot in a tight inner loop and pandas indexing is *much*
slower than numpy indexing.  We weren't using any pandas functionality for
the internal temporary data so this overhead is really unnecessary.

hande-qmc/hande,hande-qmc/hande,hande-qmc/hande,hande-qmc/hande,ruthfranklin/hande,hande-qmc/hande