Feature: code from https://github.com/DDMAL/st-gallen-lyric-extraction-toolkit integrated into doc-preproc. toolkit

When the lyric_extraction toolkit is imported, images will now have:

- extract_lyrics
- find_blackest_lines
- count_black_under_line
- count_black_under_line_points

These four methods are NOT in the GUI at this point.  Also, Nick's documentation will be integrated into the toolkit.