Tables: Implement last-row references.

Tables:  Implement last-row references.

After the failed implementation of `@0' as a reference for the last
line, this is a second attempt to provide such references.  I would
have liked to allow `@last' as the reference, that would have been
beautiful.  However, too many regular expressions directly search for
`@' followed by a number, so this is too hard to implement.

Therefore, I am now turning the last row into a row of implicitly
named fields.  From now on, $LR1, $LR2,... can be used to refer to
fields in the last row.  These names may also appear on the left hand
side of formulas.