Refactor "ocrd process" to be interoperable with OCR-D/spec#171

[kba]

• ValidationReport -> ocrd_models
• Decouple parsing and validation of tasks into OcrdWf (workflow), OcrdWfStep and OcrdWfValidator

[codecov-commenter]

Codecov Report

Merging #566 into master will increase coverage by 0.32%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #566      +/-   ##
==========================================
+ Coverage   84.15%   84.48%   +0.32%     
==========================================
  Files          48       51       +3     
  Lines        2752     2868     +116     
  Branches      538      559      +21     
==========================================
+ Hits         2316     2423     +107     
- Misses        339      347       +8     
- Partials       97       98       +1     

Impacted Files	Coverage Δ
ocrd_models/ocrd_models/report.py	100.00% <ø> (ø)
ocrd_models/ocrd_models/ocrd_wf.py	50.00% <50.00%> (ø)
...rd_validators/ocrd_validators/ocrd_wf_validator.py	94.23% <94.23%> (ø)
ocrd/ocrd/cli/validate.py	100.00% <100.00%> (ø)
ocrd_models/ocrd_models/__init__.py	100.00% <100.00%> (ø)
ocrd_models/ocrd_models/constants.py	100.00% <100.00%> (ø)
ocrd_models/ocrd_models/ocrd_wf_step.py	100.00% <100.00%> (ø)
ocrd_validators/ocrd_validators/__init__.py	100.00% <100.00%> (ø)
ocrd_validators/ocrd_validators/json_validator.py	100.00% <100.00%> (ø)
...d_validators/ocrd_validators/ocrd_zip_validator.py	100.00% <100.00%> (ø)
... and 6 more

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[kba]

This is now very obsolete, we decided to use nextflow for productive workflows. ocrd process as it-is is enough for simple workflows, so it is beyond time to close this.

[bertsky]

Agreed. Can we get something similar in core based on OtoN, though?