Add customizations to support checking +12 degC/0 FEPs violations, other minor updates

[jzuhone]

Description

This PR implements changes to support tracking, plotting, and reporting of violations of the +12 degC limit when 0 FEPs are on.

Minor updates to the regression tests are also required, to account for:

• The additions of violation durations to the violation test answers
• The changing of the limit variable names
• Adding explicit 12:00:00 to the epoch value in the test model specification to accommodate the switch to cxotime

The code changes here require acisops/acis_thermal_check#37.

Testing

• Passes unit tests on MacOS, linux, Windows (at least one required)
• Functional testing

[Gregg140]

looks good