turn return value into np.timedelta64
calculate timevalue by pd.Timedelta instead of divmod

I misinterpreted the fps I use for extracting frames with ffmpeg

not all methods and paramters were controlled

after my 1 year break in which I can not overview
the changes I try to set a new start

external function calculates contrast value and returns Image with
contrast values instead of channel value (3>1). This is calcualted with
np.histogram

reference implementation for Movie, Frames class
scatterplot contrast without plot as function in contrasts module
helper functions in helpers module