Processor helpers

[kba]

• assert_file_grp_cardinality as discussed in Ocropy processors: put derived images under output fileGrp cisocrgroup/ocrd_cis#57
• make_file_id as discussed in Ocropy processors: put derived images under output fileGrp cisocrgroup/ocrd_cis#57 with the ability to detect sequences of 3+ numbers in the ID and use that instead of n if available

[codecov-commenter]

Codecov Report

Merging #530 into master will increase coverage by 0.03%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #530      +/-   ##
==========================================
+ Coverage   79.81%   79.85%   +0.03%     
==========================================
  Files          45       45              
  Lines        2740     2760      +20     
  Branches      539      544       +5     
==========================================
+ Hits         2187     2204      +17     
- Misses        447      449       +2     
- Partials      106      107       +1     

Impacted Files	Coverage Δ
ocrd_models/ocrd_models/ocrd_file.py	91.00% <ø> (ø)
ocrd_utils/ocrd_utils/__init__.py	67.39% <75.00%> (+0.59%)	⬆️
ocrd_models/ocrd_models/ocrd_mets.py	93.13% <0.00%> (+0.98%)	⬆️

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