Added persistence map module to eitwaveutils

Merged in recent updates from wafels/master. Made a few minor changes to data_hough_detect.py and eitwaveutils.py:

- default directory for downloading eitwave data is ~/Data/eitwave/jp2/. Added check to see whether this directory exists. If not, it's created.
- when 'feed directory' is specified, added a check to return only files from that directory with the appropriate extension (e.g. '.jp2').