- add pynini dependency
- restrict parameter textequiv_level to word
  (which is the only one currently supported),
  remove default (glyph)
- add parameter for reference to rejection_weight
  (which did not exist)
- rename beam_width in FST part to pruning_weight
  (parallel to rejection_weight), add description
- replace parameter reference to lm_beam_width
  (which did not exist) to beam_width
- add FIXMEs for things that are apparently broken now
- add description of the OCR-D processor's behaviour
  to its docstring, improve README