Update Shell_Avgs to Enable Time-averaging and concatenation

[feathern]

This PR modifies the Shell_Avgs class so that it also accepts a list of multiple Shell_Avgs files. That data can either be concatenated or time-averaged as described in docstring. In addition, the class now contains an output method so that the current Shell_Avgs object structure can be saved to a new Shell_Avgs file.

Some notes:

1. Single files can still be read as they always have been.
2. This is a lot of new code. I've been using this new version of the Shell_Avgs class for a couple of years now. When doing a review, the most straightforward path is probably to test out the concatenation and time-averaging functionality.

[illorenzo7]

Looks good, thanks Nick!