Add convenient functions in mdgen.xml for child nodes, descendant
nodes etc. Tests are added for part of the functions.
The workflow for draft->wcmp13 has changed again to exclude the use of a
data-type-specific reference. It is the decoded AHL message that should
be used to fill most of the data-specific information. A reference
record is used to only fill the organisational information.
A list of data-specific information to be filled is created in
draft.clj