optimize: improve logging when performing rolling horizon

[FabianHofmann]

When running a rolling horizon, the logging was a bit poor before, this makes it better

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[codecov]

Codecov Report

Patch coverage: 62.50% and project coverage change: -0.02% ⚠️

Comparison is base (14cef6b) 78.14% compared to head (3d3428f) 78.12%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #695      +/-   ##
==========================================
- Coverage   78.14%   78.12%   -0.02%     
==========================================
  Files          26       26              
  Lines        6764     6767       +3     
  Branches     1467     1468       +1     
==========================================
+ Hits         5286     5287       +1     
- Misses       1165     1166       +1     
- Partials      313      314       +1     

Files Changed	Coverage Δ
pypsa/optimization/optimize.py	87.08% <ø> (ø)
pypsa/optimization/abstract.py	46.34% <62.50%> (-0.25%)	⬇️

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[fneum]

Just one small suggestion. LGTM.