Resolve "add interface for checking whether the default instance is defined"

Summary

Adds interface which allows the user to determine whether the default instance is defined.

Related Issues

• Closes #240 (closed)

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 successfull
• Update version info, if changes are worth mentioning (Develop section in doc/src/version_info.dox, add section if not yet included)

Closes #240 (closed)