[fitswcs] coordinates by table lookup

[Eric Greisen]

It has been suggested that table lookup will be needed to determine
coordinates on some axes.  There are radio receivers that allow the
user a nearly arbitrary list of observing frequencies.  There are data
cubes on irregular time axes.  And there are spectrometers with
calibrations so complex that table lookup has been the traditional
solution.

Unfortunately, we have not been able to come up with a scheme that
will survive general table manipulation programs, especially sorts.  I
have put together two possible schemes:
   1. Uses a vector of values in a cell in a table with an optional
      2nd vector to assist in addressing the first.
   2. Uses a column of values with 1 value per table cell and an
      optional second column to assist in addressing the first
      column.
The first uses column and row numbers explicitly, while the second
uses row number implicitly when there is not a second column.  Only
the two-column form of method 2 is impervious to changes in row order
and so I prefer it, although it may be harder to implement than the
1st method.  I use column number rather than name in the first method
since it explicitly requires row number anyway, so there is no point
in using names to try to protect against table manipulation.

The two methods are described in:

         http://www.aoc.nrao.edu/~egreisen/table_033102.ps.gz
         http://www.aoc.nrao.edu/~egreisen/table_033102.ps

and I would appreciate comments to select one (only) of the two
methods and to determine whether the table lookup is sufficiently
advanced to put it in Paper I (it is for any kind of coordinate) or
whether it will be relegated to Paper III (spectroscopy and times will
be the most common usages).

Thank you,

Eric Greisen