add reference-based midi pitch to freq

[sildater]

adds support for reference-based and tempered computation of frequencies from midi pitch information. improves over the previous version by 1) including arbitrary references 2) support of non-invertible temperament ratios tables (for arbitrary tables subtracting a maj2nd might be different from adding a min7th and subtracting an octave)

[codecov-commenter]

Codecov Report

Base: 80.59% // Head: 80.52% // Decreases project coverage by -0.07% ⚠️

Coverage data is based on head (32787a3) compared to base (b1ea05e).
Patch coverage: 9.09% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #163      +/-   ##
===========================================
- Coverage    80.59%   80.52%   -0.08%     
===========================================
  Files           63       63              
  Lines        10278    10289      +11     
===========================================
+ Hits          8284     8285       +1     
- Misses        1994     2004      +10     

Impacted Files	Coverage Δ
partitura/utils/synth.py	67.17% <9.09%> (-5.33%)	⬇️

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[manoskary]

Very nice addition. Do you think a unitest or adding an example to the docs would be useful for this?

[sildater]

I thought it was overkill, but I can add one so the coverage doesn't go down :)