lisa.datautils: Add {df,series}_dereference()
lisa.datautils: Add {df,series}_dereference()

Allows using a "pointer" column to point at values in other columns, to
achieve some sort of index-aware join.

ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa