Handle writing of datamodel definitions of dependent models properly

[tmadlener]

#358 introduced functionality to store the YAML definition of a datamodel into the produced outputfile as additional metadata. Currently only datamodels of which at least one collection is written in the file will also have their definition stored. This makes it possible to "lose" an upstream datamodel definition, which would make it impossible to recreate the dependent model from the information stored in the file.

As discussed in the PR this is mainly to keep track of this and fix it at some later point.