[SIGMON-428 ] Slop calculation corrections from Zinur

Context

• story: https://its.cern.ch/jira/browse/SIGMON-119
• task: https://its.cern.ch/jira/browse/SIGMON-428
• lhc-sm-hwc review: lhc-sm-hwc!133 (merged)

What was done

• the algorithm corrected, simplified and refactored (as described in the issue)

TODO

• consider the discrimination time

QA

• sonar: http://sonar/project/issues?branch=SIGMON-119_slope_calculation&id=lhc-sm-api&resolved=false
• tests were reviewed and adapted

Review objectives

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