Use thermal limits from JSON files, support ACIS FP in SimulateECSRun, new "attitude" argument

[jzuhone]

This PR uses the thermal limits from the JSON files in ThermalModelRunner, as well as a number of other items:

• The model specification attribute of the ThermalModelRunner class should always be a dict--so let xija handle it first before making it an attribute
• Add functionality for the ACIS FP temperature to SimulateECSRun check to see how much of the measurement will be "cold" and what temperature it reaches.
• Every possible option for inputting spacecraft attitude into SimulateECSRun is now boiled down to a single "attitude" parameter. Options are:
  • (pitch, off nominal roll) tuple (constant state)
  • attitude quaternion (constant state)
  • load name (for ECS runs during vehicle-only loads)
• A new test for the new ACIS FP functionality in SimulateECSRun was added.
• Various doc fixes

Hard dependence on acisops/acis_thermal_check#45, soft dependence on sot/xijafit#14.

Tests in the ACISpy test suite pass.