Fully qualify relation return types in generated .cc files

[tmadlener]

BEGINRELEASENOTES

• Fully qualify return types for OneToOneRelation getters in generated .cc file for objects and Const objects. This fixes a bug described in Parse multiple yaml files at the same time #168 (comment) and now allows to mix different namespaces in the generated code. This allows to more easily extend already existing datamodels by compiling and linking against them.

ENDRELEASENOTES

Additionally, cleaned up the templates for the OneToOneRelation setters.

[tmadlener]

CI seems to be having problems with the github-action-cvmfs, which seems to be caused by problems accessing the cvmfs package repository at cern: https://github.com/AIDASoft/podio/pull/175/checks?check_run_id=1806087593#step:3:60

[petricm]

only a transient connection problem

[tmadlener]

triggering CI again.