Ocrdfile cleanup

[kba]

Removes unused argument instance and adds ID as a constructor argument.

Required by upcoming PR, so will merge if tests pass.

[codecov-commenter]

Codecov Report

Merging #504 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #504   +/-   ##
=======================================
  Coverage   79.53%   79.53%           
=======================================
  Files          41       41           
  Lines        2516     2516           
  Branches      488      488           
=======================================
  Hits         2001     2001           
  Misses        424      424           
  Partials       91       91           

Impacted Files	Coverage Δ
ocrd_models/ocrd_models/ocrd_file.py	90.90% <100.00%> (ø)

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