wrap more Python image processing libraries

[bertsky]

Candidates for low-effort wrappers:

• pgmagick, instead of using convert with ocrd-preprocess-image
• pyleptonica, perhaps mimicking some of Tesseract's better usage (like h/v-line detection or flip detection)
• more from OpenCV (but ocrd_cis already wraps its binarization and morphology functions)

[bertsky]

also:

• deskewing based on skimage's Hough transform