Add XSD validators for PAGE and METS, #449

[kba]

Adds XSD validators, bundles METS, XLINK and PAGE schemas. XSD validation is part of workspace validation unless page_xsd and/or mets_xsd are skipped.

[codecov-io]

Codecov Report

Merging #470 into master will increase coverage by 0.18%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #470      +/-   ##
==========================================
+ Coverage   80.23%   80.42%   +0.18%     
==========================================
  Files          40       43       +3     
  Lines        2383     2426      +43     
  Branches      432      434       +2     
==========================================
+ Hits         1912     1951      +39     
- Misses        398      402       +4     
  Partials       73       73              

Impacted Files	Coverage Δ
...d_validators/ocrd_validators/xsd_mets_validator.py	71.42% <71.42%> (ø)
ocrd_validators/ocrd_validators/xsd_validator.py	90.47% <90.47%> (ø)
ocrd_validators/ocrd_validators/__init__.py	100.00% <100.00%> (ø)
ocrd_validators/ocrd_validators/constants.py	100.00% <100.00%> (ø)
...d_validators/ocrd_validators/xsd_page_validator.py	100.00% <100.00%> (ø)

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[codecov-commenter]

Codecov Report

Merging #470 into master will increase coverage by 0.07%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #470      +/-   ##
==========================================
+ Coverage   79.53%   79.60%   +0.07%     
==========================================
  Files          41       44       +3     
  Lines        2516     2559      +43     
  Branches      488      490       +2     
==========================================
+ Hits         2001     2037      +36     
- Misses        424      431       +7     
  Partials       91       91              

Impacted Files	Coverage Δ
...d_validators/ocrd_validators/xsd_mets_validator.py	71.42% <71.42%> (ø)
ocrd_validators/ocrd_validators/xsd_validator.py	90.47% <90.47%> (ø)
ocrd_validators/ocrd_validators/__init__.py	100.00% <100.00%> (ø)
ocrd_validators/ocrd_validators/constants.py	100.00% <100.00%> (ø)
...d_validators/ocrd_validators/xsd_page_validator.py	100.00% <100.00%> (ø)
ocrd/ocrd/resolver.py	93.10% <0.00%> (-3.45%)	⬇️
ocrd_models/ocrd_models/ocrd_file.py	90.90% <0.00%> (ø)

---

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[bertsky]

Learned a lot about lxml and the class method pattern here!