clean up access to list-of-list dataframe, comment that
we need subscript verification, and change var name to
reflect this is a pre-df not a df.

Signed-off-by: AJ Rossini <blindglobe@gmail.com>
ROW and COLUMN should only be in the lisp-matrix or GSLL side.  Dataframes are Case/Variable or similar domain
specific description!

Signed-off-by: AJ Rossini <blindglobe@gmail.com>