Dependency of calibration.calibrator on digital_twin module

[twright]

The calibration.calibrator module has a dependency on digital_twin.simulator.plant_simulator.PlantSimulator4Params from the example_digital-twin_incubator repository on account of the import

from digital_twin.simulator.plant_simulator import PlantSimulator4Params

on line 7 of calibration/calibrator.py.

This prevents tests such as SelfAdaptationTests.test_run_self_adaptation from being run independently of the digital_twin module.