use reduceat when aggregating to improve performance
fix mask bug
Special case np.mean. Check for MaskesdColumn.
Added tests for reduceat aggregation
Mention the substitution of ufuncs with reduceat
Update doc and CHANGES
Change IPython HTML table repr to show all columns
Merge pull request #2625 from luzuku/group-aggregate

Use reduceat() in group aggregation where possible.  Closes #1546.
Merge pull request #2651 from taldcroft/table-html-repr

Change IPython Table HTML repr to show all columns
Fixed kernel multiplication with  numpy floats