Kern import fix

[manoskary]

This PR addresses issues #158 and #159 and other minor bug fixes.

[codecov-commenter]

Codecov Report

Base: 80.59% // Head: 80.55% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (d5fa5f9) compared to base (d1bee81).
Patch coverage: 46.15% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #160      +/-   ##
===========================================
- Coverage    80.59%   80.55%   -0.05%     
===========================================
  Files           63       63              
  Lines        10278    10289      +11     
===========================================
+ Hits          8284     8288       +4     
- Misses        1994     2001       +7     

Impacted Files	Coverage Δ
partitura/score.py	79.55% <ø> (ø)
partitura/utils/music.py	68.22% <40.00%> (-0.22%)	⬇️
partitura/io/importmusicxml.py	74.04% <42.85%> (-0.36%)	⬇️
partitura/io/importkern.py	85.80% <100.00%> (ø)

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