an output can only be linked to a single report

[nnhjy]

One can use the unit-test system to reproduce the issue:

SpineOpt.jl/test/data_structure/postprocess_results.jl

Line 239 in d894790

#FIXME: Another report with the same output will fail the test by an error

SpineOpt.jl/test/run_spineopt_multi_stage.jl

Line 52 in d894790

#FIXME: Uncomment either of the following lines will fail the test.

If this design is intended, there needs something to prevent the user from linking multiple report entities to the same output entity.