coordinates_for_segment: add optional parent_element…

[bertsky]

…to ensure that the given polygon is fully contained within that parent's polygon.

Fixes #489.

But please don't merge yet, I have another, better idea

[codecov-commenter]

Codecov Report

Merging #492 into master will decrease coverage by 1.09%. The diff coverage is 18.18%.

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
- Coverage   80.12%   79.03%   -1.10%     
==========================================
  Files          40       40              
  Lines        2390     2485      +95     
  Branches      443      482      +39     
==========================================
+ Hits         1915     1964      +49     
- Misses        403      433      +30     
- Partials       72       88      +16     

Impacted Files	Coverage Δ
ocrd_utils/ocrd_utils/__init__.py	62.31% <18.18%> (-3.70%)	:arrow_down:
ocrd_validators/ocrd_validators/page_validator.py	77.19% <0.00%> (-4.06%)	:arrow_down:
ocrd/ocrd/cli/workspace.py	68.86% <0.00%> (-2.94%)	:arrow_down:
ocrd_models/ocrd_models/ocrd_mets.py	93.92% <0.00%> (-0.76%)	:arrow_down:
ocrd/ocrd/workspace.py	58.02% <0.00%> (ø)

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