Feature/basic align

[laurejt]

Associated Issue(s): resolves #7, #8

Changes in this PR

• Update handling of spans attribute for DocSpans to ensure immutability
• Add library spans/match of some basic methods for span matching
• Add SpanAlignment class
• Add custom function types CheckSpanPair, ScoreSpanPair for readablility
• Add library align with three alignment strategies

Reviewer Checklist

• Unit tests run locally
• Code documentation is legible and sufficient

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f0f9709) to head (d15c79f).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop        #9    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
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  Files            3         7     +4     
  Lines          258       545   +287     
  Branches        13        36    +23     
==========================================
+ Hits           258       545   +287     

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[rlskoeser]

Everything looks reasonable to me. Made some suggestions about places you could clarify some of the logic in comments/docs a bit