pytables: head on column
add t.column.head() to comprehensive compute test
Change scalar_symbol into expr
Merge pull request #680 from mrocklin/pytables-column-head

Pytables column head
Merge pull request #682 from cpcloud/remove-scalar-symbol

Change scalar_symbol into expr
Run ColumnWise test
First attempt
Rename
post_compute dispatch on ColumnWise
Should be get not pop
Implement mod and raise on ops not found
Code cleanup
Correct the comparison results
Add expr property for slightly more uniform usage
Allow more columnwise in summary ops
Test more complicated arithmetic
max and min for columnwise summary
Don't dispatch on compute_sub
Docstring for compute_sub in compute/mongo.py
Wait for #682