Unit tests for io module

[ns-rse]

Whilst looking through the code base to resolve #70 I noticed that test_io.py::test_load_config() appears to test
multiple functions, some of which are conditional on the use_default value. Functions tested are...

Function	use_default value triggers when
io.load_config_impl()	True
io.config_wrapper.unflatten()	True and False
io.yaml.safe_load()	False

Ideally there should be unit tests in place of their own to check each of these functions works independent of this
integration test.