Spectral DM document update

[Jonathan McDowell]

I've incorporated the recommendations from Moscow
into the Spectrum document; it's at
 http://hea-www.harvard.edu/~jcm/vo/docs/spec98/spec98d.pdf
There have been no changes to the list of UTYPEs but I have
added mandatory/recommended/optional markers, and altered the
FITS part following comments from STScI. 
 I have not yet addressed the issue of the XML schema serialization
of the data segment: the two alternate versions include a simple
XML <Point>.... </Point> which is very verbose, and a more compact
attribute-laden Point which was developed following Lazlo's example
in the JHU service. However Lazlo himself suggested that we replace
this with arrays like <Flux>1.2 1.3 1.4 ... </Flux> where all data
point attributes are in separate arrays, instead of having
Point No. 1 (flux, err, spco, ...) then Point No. 2 (flux, err, spco...)
etc. I didn't get much feedback in Moscow about whether this was
considered by others to be a good idea, since the email discussion
got sidetracked onto the format-specifying issue that Lazlo raised
in the same email.
 Other changes in the document are mainly clarifications
and extra definitions of existing elements, and small changes to the XML.
(In particular, Doug, there should be no impact on the SSA protocol).

  - Jonathan