Merge ERLE functions

[oscarbailey-xmos]

There is already a function called get_erle() in audio_utils.py. I would merge the two functions rather than duplicating code.

There are currently four different ERLE functions that I know of:

• Keith's ERLE for AEC handoff https://xmosjira.atlassian.net/wiki/spaces/HYD/pages/1263927297/Handoff+Criteria#HandoffCriteria-ERLE
• The get_erle function in audio_utils.py
• The ERLE calculation in the sw_xvf3510 host app
• The new erle function that this PR has (test_wav_erle)

It would be good to verify that they are all doing the same thing and merge the python ones but I think we could have that as a separate piece of work for the next version of audio_test_tools

Originally posted by @oscarbailey-xmos in #88 (comment)