Definition of dataproduct_type terms for the radio domain and Appendix for use cases

[alle-ira]

Currently allowed terms for dataproduct_type do not permit a proper data characterisation of SD data. The conclusion of the revision of the reserved list of terms for dataproduct_type currently under discussion within IVOA will allow the identification of an appropriate list of terms. The discussion will benefit from (and its conclusions may be affecte by) the addition of an Appendix listing use cases for both SD and VLBI data to the ObsCore Radio Extension document.