Use ACA drift model from chandra_models JSON file

[taldcroft]

Description

This updates the ACA drift model module so that it takes the parameters from a JSON file instead of having them hardwired into the code.

By default the parameters are in $SKA/data/chandra_models/chandra_models/aca_drift/aca_drift_model.json. This can be overridden for testing by setting the ACA_DRIFT_MODEL_PATH environment variable.

Interface impacts

No external impacts, but it does require promotion of:

• https://github.com/sot/chandra_models/pull/95

Testing

Unit tests

Put the new drift model file from https://github.com/sot/chandra_models/pull/95 into my Mac $SKA/data/chandra_models repo (as an unversioned file) and ran unit tests successfully. Inserted a temporary print statement to confirm that the expected file was being used.

• [x] Mac (with new unit test)

Independent check of unit tests by Jean

• [x] Linux - unit tests run with ACA_DRIFT_MODEL_PATH set to model file in https://github.com/sot/chandra_models/pull/95 as noted.

Functional tests

New unit test.

[jeanconn]

Also, I think the updated unit tests are a functional test.

[taldcroft]

@jeanconn - I ran the unit tests for record as now documented.