Fix Issue #248

[CarlosCancino-Chacon]

This pull request closes issue #248, i.e., fixes the behavior of the clip_note_off argument in slice_ppart_by_time.

The pull request consists of

• The bug fix itself
• An upgraded test checking both the clip_note_off and reindex_notes arguments of slice_ppart_by_time

[codecov-commenter]

Codecov Report

Patch coverage: 91.66% and project coverage change: +0.04 🎉

Comparison is base (59cbe3a) 85.00% compared to head (6731eeb) 85.04%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #253      +/-   ##
===========================================
+ Coverage    85.00%   85.04%   +0.04%     
===========================================
  Files           72       72              
  Lines        12653    12667      +14     
===========================================
+ Hits         10756    10773      +17     
+ Misses        1897     1894       -3     

Impacted Files	Coverage Δ
partitura/utils/music.py	74.48% <50.00%> (+0.30%)	⬆️
tests/test_utils.py	98.13% <100.00%> (+0.12%)	⬆️

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[manoskary]

Minor correction to address issue #248 by adding missing argument for cutoff.
Also some addition to test utils and minor documentation typo corrections.