RFE add support for PRIMARYKEY

[glemson]

The following VOTable annotation segment declares a PRIMARYKEY. In the Source instance created by rama this is left out.
PRIMARYKEY is very important to distinguish ObjectType instances, particularly in certain flattened situations or ORM FK->PK mapping:

  <TEMPLATES tableref="T1622026678915_1">
         <INSTANCE dmtype="sample:catalog.Source" ID="T1622026678915_718">
             <PRIMARYKEY>
                 <PKFIELD>
                     <COLUMN dmtype="ivoa:string" ref="oidsaada_100"/>
                 </PKFIELD>
             </PRIMARYKEY>