Basic img_proc module revision and re-org using API option

Reorganized the thresholding, histogram, morphology and filter operations so that all direct imports are removed and now loaded via the api.

Remaining tools to follow.
I dont understand what this means. 

If I want to use these filters i would have to 
`from scipy.ndimage import gaussian_filter, median_filter, etc...` 
instead of 
`from skxray.api.ff_image import gaussian_filter, median_filter, etc...`

Is that correct?