ENH: Restrict selection predicates to be ColumnElements
ENH: Restrict selection predicates to be ColumnElements

This is a tighter restriction because we don't want selects for the predicate.

cpcloud/blaze,ContinuumIO/blaze,cowlicks/blaze,cowlicks/blaze,cpcloud/blaze,ContinuumIO/blaze,ChinaQuants/blaze,ChinaQuants/blaze