xtype: similarity to VODataService's externalType

[Ray Plante]

Hi VOTablers,

As discussed in the last VOTable session in Strasbourg, we are considering 
adding a new FIELD attribute called xtype (or exttype or externalType). 
As I mentioned, this is very similar to an attribute availble in 
VODataService for describing a table column.

For reference, here's how the VODataService Document 
(http://www.ivoa.net/Documents/latest/VODataService.html) describes 
them...

   More descriptive information about the type can be provided via the
   extendedType and extendedSchema, which provide an alternate data type
   name. It's expected that this name will only be understood by a special
   subset of applications. The name given in the element content, then,
   represents a more commonly understood "fall-back" type.

     extendedType  Value type: 	     string: xs:string.
                   Semantic Meaning:  the data value represented by this
                                      type can be interpreted as of a
                                      custom type identified by the value
                                      of this attribute.
                   Occurrences: 	     optional
                   Comments:          The name implies a particular
                                      expected format for the data
                                      value that can be parsed into a
                                      value in memory.

                                      If an application does not recognize
                                      this extendedType, it should attempt
                                      to handle value assuming the type
                                      given by the element's value.
                                      "string" (or its equivalent) is a
                                      recommended default type.

                                      This element may make use of the
                                      extendedSchema attribute and/or any
                                      arbitrary (qualified) attribute to
                                      refine the identification of the
                                      type.

   extendedSchema  Value type:        URI: xs:anyURI.
                   Semantic Meaning:  An identifier for the schema that
                                      the value given by the extended
                                      attribute is drawn from.
                   Occurrences:       optional
                   Comments:          This attribute is normally ignored if
                                      the extended element is not present.

hope this is helpful for reference.

cheers,
Ray