Model zoo

[kba]

Still WIP, intermediate state for discussion

• all logic related to loading ML models moved to EynollahModelZoo
• all models can have variants, such as light. Default is ''
• model overrides are a three-string tuple (model_category, model_variant, model_filename)
• Eynollah_ocr moved into a separate module

TODO:

• better data structure for the defaults (including documentation e.g.)
• apply to remaining runners (i.e. classes that have a run method and are called via CLI)
• [ ] Factor out a common base class for all runners not required for this PR
• make --model* options for the top-level eynollah click group