Introduce stage__output

[manuelma]

To simplify fixing outputs for all entities, rather than listing them all under, say, stageoutputunit.

Checklist before merging

• [ ] Documentation is up-to-date
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted according to SpineOpt's style
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Project coverage is 56.43%. Comparing base (914304f) to head (2575de6). Report is 2 commits behind head on v0.7.2.x-release.

Files with missing lines	Patch %	Lines
src/run_spineopt_basic.jl	0.00%	9 Missing :warning:
src/run_spineopt.jl	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v0.7.2.x-release #1094 +/- ## ==================================================== - Coverage 56.55% 56.43% -0.13% ==================================================== Files 141 141 Lines 3715 3723 +8 ==================================================== Hits 2101 2101 - Misses 1614 1622 +8 ```

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