[SIGMON-251] use U_REF_N1 signal

Context

• task: https://its.cern.ch/jira/browse/SIGMON-251
• goal: to check if the U_REF_N1 signal, which is queried instead of U_DIODE_RQx for certain magnets (see https://its.cern.ch/jira/browse/SIGMON-179) is used properly by the analysis' methods

What was done

• the logic of obtaining diode signal name (and its source) moved to the Mapping Metadata class
• the filtering criteria in DiodeLeadResistanceAnalysis#plot_current_voltage_nxcals method changed

TODO

• refactoring of the DiodeLeadResistanceAnalysis https://its.cern.ch/jira/browse/SIGMON-262

QA

• sonar: http://sonar.cern.ch/dashboard?branch=SIGMON-251_use_U_REF_N_signal&id=lhc-sm-api
• code was unit tested
• notebooks passed: https://gitlab.cern.ch/LHCData/lhc-sm-hwc/-/jobs/17967071

Review objectives

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