Updated AZ_Avgs class to allow concatenation and time-averaging.

[feathern]

This PR modifies the AZ_Avgs class so that it also accepts a list of multiple AZ_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 AZ_Avgs object structure can be saved to a new AZ_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 AZ_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 @feathern

[illorenzo7]

@feathern It looks like the AZ_Avgs routine now prints "the_file" any time the class is instantiated from a binary file. I believe that was added in this pull request. Is this behavior strictly necessary or could I make a pull request to remove it? The only reason I care is that it's messing up some of the output from my personal routines (especially the parallelized ones).

[feathern]

Oops. Yes, please do go ahead and submit a PR with the fix. This was an oversight.