Align attributes and return types

[Illviljan]

I'm attempting to move the data in DcmReader to a xarray dataset but it is a annoying having to deal with the different return types and missing attributes.

• DcmDistribution.values return list instead of dict.
• DcmFunction.values does not exist. Could have been an empty list instead.

My suggestion is to try to add a base class that DcmDistribution etc is inheriting from where self.values only contain the WERT values, axis values (or coordinates) are moved to a self.coords.

I made an example PR here, #7.
High level ideas:

• Inherit/use only a single dcm class, reduces typo risks etc, Risk of getting too much clutter.
• Follow typical conventions popular in data analysis tools like numpy /xarray
  • Use self.shape instead of x_dimension y_dimension. -> Following numpy.array.shape standard.
  • Use list of list for WERT values. -> Easy to initialize numpy.array then.
  • Use tuple of lists for axis values. -> Matching order of numpy.array.shape-tuples
  • Move remaining metadata to a self.attrs dict. -> In similar fashion as xarray/pandas
• A lot of type hints to clarify intention of methods and attributes.