bugfix for midi scores with ts only in one track

[fosfrancesco]

Some midi scores, like the one produced by Musescore, have 2 tracks, one for each hand, but time signatures only in one track. The other track is then assumed to be 4/4 and this is completely scrambling the note positions if this is untrue.

Now, if only one track has time signatures, they are shared across all tracks.
For the (very very uncommon) case where the midi file maker wanted to have different time signatures for each track, we assume they did specify at least one time signature for each track, so this bugfix won't change anything

[codecov-commenter]

Codecov Report

Patch coverage: 75.86% and project coverage change: +0.11% 🎉

Comparison is base (5fe996d) 85.13% compared to head (5f18fd8) 85.24%.
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #308      +/-   ##
===========================================
+ Coverage    85.13%   85.24%   +0.11%     
===========================================
  Files           80       80              
  Lines        13663    13678      +15     
===========================================
+ Hits         11632    11660      +28     
+ Misses        2031     2018      -13     

Files Changed	Coverage Δ
partitura/io/musescore.py	27.47% <0.00%> (ø)
partitura/utils/music.py	74.20% <ø> (ø)
partitura/io/importmei.py	92.16% <77.77%> (ø)
partitura/io/importmidi.py	87.50% <100.00%> (+3.55%)	⬆️
tests/__init__.py	100.00% <100.00%> (ø)
tests/test_midi_import.py	98.91% <100.00%> (+0.04%)	⬆️

... and 2 files with indirect coverage changes

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