Introduce stage__output

[manuelma]

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

Checklist before merging

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

[codecov]

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 ⚠️
src/run_spineopt.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                 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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