rename contrast classes

accordingly to the scope of the class which now is not a specific
contrast but a way to represent contrast. This is more consistent with
what the data represents and how to work with the data.

testing/bootstrap.py

 from itten.movie import Movie
-from itten.contrasts import LightDark
+from itten.contrasts import VHistStack
 
 movie = Movie(prefix='rec_', folder='../DHd-2017/Data/Frames/Rec/')
-cont = LightDark(movie._frames)
+cont = VHistStack(movie._frames)