[SIGMON-546] Implement pli2.e3 analysis class

Context

• Agnieszka implemented the notebook for the pli2_e3 analysis class (available here https://its.cern.ch/jira/browse/SIGMON-544), but the analysis logic needed to be extracted.
• link to the readme: https://gitlab.cern.ch/LHCData/lhc-sm-analysis/lhc-sm-powering/-/tree/SIGMON-545_adding_pli2.e3/lhcsmapi/api/analysis/powering/pli2.e3?ref_type=heads

What was done

• implement pli2.e3 analysis
• implement corresponding notebook

TODO

• Refactor the query for the spa and flt timestamps once the metadata in available through sigmon

QA

• code was unit tests
• acceptance tests were written

Review objectives

• ? assessment of code functionality
• ? assessment of code quality