Resolve "Catch definition of multiple src per tgt early"

Summary

The format of the YAML configuration file currently allows the definition of multiple source grids in a single couple entry. Since this would create multiple couples with different source fields for a single target field, which is not allowed, this is a bug.

Related Issues

• Closes #275 (closed)

Documentation

Checklist

• All code changes are covered by unit tests
• Code coverage did not deteriorate
• Code follows Style Guide
• Documentation updated, if necessary
• WIP: Manual running of configure/build jobs was successful
• Update changelog, if changes are worth mentioning (Develop section in doc/src/changelog.dox, add section if not yet included)

Closes #275 (closed)