Harmonize all string-representations of objects

[danielhuppmann]

I thought that having the id first and without clear identification of its meaning wasn't really helpful to average users like me... Tried to do this consistently through all similar items.

[codecov]

Codecov Report

❌ Patch coverage is 24.00000% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.7%. Comparing base (d4aba97) to head (83e3cb6).

Files with missing lines	Patch %	Lines
ixmp4/core/optimization/equation.py	0.0%	1 Missing ⚠️
ixmp4/core/optimization/indexset.py	0.0%	1 Missing ⚠️
ixmp4/core/optimization/parameter.py	0.0%	1 Missing ⚠️
ixmp4/core/optimization/scalar.py	0.0%	1 Missing ⚠️
ixmp4/core/optimization/table.py	0.0%	1 Missing ⚠️
ixmp4/core/optimization/variable.py	0.0%	1 Missing ⚠️
ixmp4/data/iamc/variable/dto.py	0.0%	1 Missing ⚠️
ixmp4/data/meta/dto.py	0.0%	1 Missing ⚠️
ixmp4/data/model/dto.py	0.0%	1 Missing ⚠️
ixmp4/data/optimization/equation/dto.py	0.0%	1 Missing ⚠️
... and 9 more

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #252   +/-   ##
=====================================
  Coverage   96.7%   96.7%           
=====================================
  Files        221     221           
  Lines       9821    9821           
=====================================
  Hits        9500    9500           
  Misses       321     321           

Files with missing lines	Coverage Δ
ixmp4/core/iamc/variable.py	98.3% <100.0%> (ø)
ixmp4/core/model.py	96.8% <100.0%> (ø)
ixmp4/core/region.py	97.0% <100.0%> (ø)
ixmp4/core/run.py	96.9% <ø> (ø)
ixmp4/core/scenario.py	96.8% <100.0%> (ø)
ixmp4/core/unit.py	97.0% <100.0%> (ø)
ixmp4/data/checkpoint/dto.py	90.0% <ø> (ø)
ixmp4/data/docs/dto.py	100.0% <100.0%> (ø)
ixmp4/core/optimization/equation.py	97.8% <0.0%> (ø)
ixmp4/core/optimization/indexset.py	96.3% <0.0%> (ø)
... and 17 more

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[meksor]

Looks good!