Add pandas to pip installation

[HealthyPear]

If installing only using pip it now required by both protopipe-TRAINING and protopipe-DL2 that pandas is installed together with ctapipe and pyirf

TODO:

• test pure pip installation in the CI and not only the one based on the conda environment file

[codecov]

Codecov Report

Merging #138 (6e965e1) into master (f3ccc76) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #138   +/-   ##
=======================================
  Coverage   60.35%   60.35%           
=======================================
  Files          23       23           
  Lines        2101     2101           
=======================================
  Hits         1268     1268           
  Misses        833      833           

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