Refactor: Use named tuples for Subject and AnalysisHit

[osma]

No description provided.

[coveralls]

Coverage increased (+0.06%) to 95.398% when pulling edbeae8 on namedtuple into dc37e4f on master.

[codecov]

Codecov Report

Merging #87 into master will increase coverage by 0.05%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   95.34%   95.39%   +0.05%     
==========================================
  Files          29       29              
  Lines        1138     1130       -8     
==========================================
- Hits         1085     1078       -7     
+ Misses         53       52       -1

Impacted Files	Coverage Δ
annif/backend/tfidf.py	100% <ø> (ø)	⬆️
annif/rest.py	33.33% <0%> (ø)	⬆️
annif/backend/http.py	100% <100%> (ø)	⬆️
annif/project.py	93.38% <100%> (ø)	⬆️
annif/hit.py	100% <100%> (+14.28%)	⬆️
annif/backend/dummy.py	100% <100%> (ø)	⬆️
annif/corpus/subject.py	82.05% <75%> (-1.29%)	⬇️

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