allow 0 operating- and downtime

[jrasko]

Description

Fix bug that forces powerplants to have a non-zero min_down_time and min_operating_time

Checklist

• Changes noted in release notes (if any)
• Consent to release this PR's code under the GNU Affero General Public License v3.0

[kim-mskw]

PR template please @jrasko

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.31%. Comparing base (c008633) to head (fdfa2c2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #704   +/-   ##
=======================================
  Coverage   81.31%   81.31%           
=======================================
  Files          54       54           
  Lines        8032     8032           
=======================================
  Hits         6531     6531           
  Misses       1501     1501           

Flag	Coverage Δ
pytest	81.31% <100.00%> (ø)

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[kim-mskw]

Fixes bug that was introduced after the last release, so no release notes needed.