Reading a YAML configuration should return a helper class instead of a dict

When loading a YAML configuration file, a dict is returned.

Example

>>> import pyxel
>>> cfg = pixel.io.load(input_filename)
>>> cfg
{'pipeline': ..., 'simulation': ..., 'ccd_detector': ..., 'cmos_detector': ...}

It would be better to return a helper class.

Example

>>> import pyxel
>>> cfg = pixel.io.load(input_filename)
>>> cfg.pipeline
...
>>> cfg.simulation
...
>>> cfg.ccd_detector
...