New plotting routine in eitwaveplot, added base map difference module to eitwaveutils, and tweaks to data_hough_detect

Here are the main changes for the pull request:

EITWAVEUTILS:
 - added a base map difference function
 - corrected a bug in params module where latitude and longitude coordinates from HEK were assigned the wrong way round.

EITWAVEPLOT
 - modifications to velocity_polyfit function. Now second order, uses actual time information from input maps
 - added a width_vs_longitude plot function

DATA_HOUGH_DETECT
 - some cleaning up (removed some duplicate calls, e.g. to eitwaveutils.map_unravel
 - added diff_type as an argument for main. Default is 'running', can also choose 'base'
 - temporary fix to only grab long exposure maps, and throw away short exposure images. Short exposures were causing problems with the difference mapping.