New features for config classes

[thibaultdvx]

Suggestion of a small adjustments for config class:

• The config class (eg. RescaleIntensityConfig) will automatically get the default values from the associated class (e.g. torchio. RescaleIntensity) when instantiated:

>>> config = RescaleIntensityConfig(out_min_max=0, in_min_max=12)
>>> config
RescaleIntensityConfig(masking_method=None, out_min_max=0.0, percentiles=(0.0, 100.0), in_min_max=12.0, name='RescaleIntensity')

• One can easily retrieve the associated object with the method get_object:

>>> config.get_object()
RescaleIntensity(out_min_max=0.0, percentiles=(0.0, 100.0), masking_method=None, in_min_max=12.0)

@camillebrianceau tell me what do you think. If it looks interesting to you I will make the changes for all config classes.