Static data iosource

[toqduj]

Added an IoSource for static data loaded in from a yaml file. Has a path-like interface, see associated tests. Also had to fix MessageHandler to get it to work (or I misunderstood).

Can be used like this:

filepath = Path("../src/modacor/tests/io/static_data/static_data_example.yaml")
sm_iosource = StaticData(source_reference='defaults')
sm_iosource._load_from_yaml(filepath)  # you can add multiple if you want, will update (!) internal dict. 
sm_iosource.get_data('probe_properties/wavelength/value')  # always an np.ndarray, uses additional internal cache for speed in future access of the same data. 
sm_iosource.get_static_metadata('probe_properties/wavelength/unit')  # any type, here 'nm'

[toqduj]

YAMLLoader can be used to load values from YAML files. These can be metadata such as wavelengths and other machine information.