[SIGMON-222] Support many EE references

Context

• ticket: https://its.cern.ch/jira/browse/SIGMON-222
• lhc-sm-hwc MR: lhc-sm-hwc!60 (merged)

What was done

• Support for many reference EE events added (in the same way as it was already done for the discharges)
• Reference class removed, all the methods moved to lhcsmapi.reference.py and lhcsmapi.analysis.comparison
• CSVs with the reference data moved to lhcsmapi.resources.reference
• pkgutil used to read the CSVs (instead of joining the paths)
• small refactoring of the references methods and tests
• 2 new methods added to PcQuery (get_fgc_timestamp_ref and get_ee_timestamp_ref) replacing get_timestamp_ref(csv_column_name)

TODO

• ?

QA

• code was unit tested
• notebooks were checked

Review objectives

• assessment of code functionality
• assessment of code quality
• identifying potential refactorings / logic common to other projects
• knowledge sharing