[SIGMON-309] SignalMetadata

Context

• task: https://its.cern.ch/jira/browse/SIGMON-309
• lhc-sm-hwc MR: lhc-sm-hwc!94 (merged)

What was done

• default behaviour of SignalMetadata changed: if no timestamp is provided, the most recent metadata is returned
• SignalMetadata partially refactored: unused methods removed, module renamed
• tests improved

To review

• signal_metadata.py
• test_signal_metadata.py
• check if timestamp argument is passed to the signal_metadata's functions (if possible)

TODO

• ? (it would be nice to remove jsons one day)

QA

• http://sonar.cern.ch/dashboard?branch=SIGMON-309_timestamp_arg_required_in_the_metadata&id=lhc-sm-api (coverage is 96%)
• tests written
• notebooks passed: https://gitlab.cern.ch/LHCData/lhc-sm-hwc/-/jobs/19638237

Review objectives

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