Simplification of code, use cxotime, fixing bugs with violations reporting

[jzuhone]

Description

This PR makes a number of changes which simplifies code, reduces duplication, and fixes a bug:

• The acis_thermal_check refactor in Refactoring of limit handling, allowing for more customizations, use cxotime acis_thermal_check#37 allows for further code removal and simplification in ACISFPCheck.
• Complete refactor and simplification of the code which determined the "OBSID intervals", which removes a lot of code which is no longer being used.
• Fixes a bug which sometimes resulted in thermal violations being reported during maneuvers.
• We now explicitly check for thermal violations in between the start and stop of science for a given observation.
• The paint_perigee function, which plotted perigee passages on prediction plots, and additional required code for this purpose, has been moved to acis_thermal_check, since all models will now use it.
• The index template file has been removed, as the differences from the one in acis_thermal_check were minor and the functionality has been merged into that one.
• cxotime is now used instead of Chandra.Time.

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