Adding features to Group class

[Ameyanagi]

@newville
I am working on adding a feature to the Group class that allows iteration and getting attributes in a similar manner as a dictionary.

Please look at the notebook attached for an example of how it helps to work on multiple spectra.
examples/Jupyter/Iteration_of_Group.ipynb

I believe this will provide a simple and straightforward way of dealing with multiple spectra. (for example, dealing with Athena project files.)

I am currently using a manual implementation of several methods, but I think there is an ideal way to use dataclasses or other modules. I would be happy to hear any kind of recommendations and improvements.

The current implementation is only done for the parent Group class and Athena project class.