HoleSet - supporting missing data in Goose

["Asger K. Alstrup Nielsen" <alstrup diku dk>]

Hi!

I've added the start of a wrapping class that will transform
any existing DataSet derived from DataSet_typed (or more
generally, any that provides the "dataset_type" typedef)
into a new DataSet that supports missing data.

I've only tested this stuff on Visual C++ 6.0, where it
seems to work.  I'm using templates and templatized
derivation, which is fairly "modern".

I'm just wondering whether this stuff works with egcs..
Could somebody please test?  If it doesn't, I'll just
provide the HoledRealSet by hand, because this is
what I specifically need right now.  Of course, I'd prefer
the stuff that's in right now, because I think it's
a pretty cool way to do it ;-)

Greets,

Asger