mmif.__getitem__() now works with short annotation ID

[keighrim]

fixes #279.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.37%. Comparing base (6cec53f) to head (90ec32f).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #282      +/-   ##
===========================================
- Coverage    91.41%   91.37%   -0.05%     
===========================================
  Files           10       10              
  Lines         1282     1287       +5     
===========================================
+ Hits          1172     1176       +4     
- Misses         110      111       +1     

Flag	Coverage Δ
unittests	91.37% <100.00%> (-0.05%)	⬇️

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