Add base device state model tests, and fix fatal error bug

.release

README.md

docs/source/SKABaseDevice.rst

docs/source/images/device_state_diagram.png

src/ska/base/base_device.py

src/ska/base/release.py

tests/conftest.py

tests/test_base_device.py

tests/test_subarray_device.py

tests/test_subarray_state_model.py