Add pandas as direct dependency

[danielhollas]

pandas is used in several widgets to generate HTML tables. While the ultimate goal is to replace it with a lighterweight dependency, for now we declare it as direct dependecy. Previously, it was defined only transitively via optimade_client, which is now turned into optional optimade extras.

Closes #588

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.04%. Comparing base (826ad43) to head (2d7033a).
Report is 4 commits behind head on master.

❗ Current head 2d7033a differs from pull request most recent head b90bf20. Consider uploading reports for the commit b90bf20 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
- Coverage   96.17%   96.04%   -0.13%     
==========================================
  Files          11       11              
  Lines        1177     1189      +12     
==========================================
+ Hits         1132     1142      +10     
- Misses         45       47       +2     

Flag	Coverage Δ
python-3.10	96.04% <100.00%> (-0.13%)	⬇️
python-3.9	96.04% <100.00%> (-0.13%)	⬇️

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[superstar54]

Thanks for the fix!