feat: add mergeLabels() method to IntervalTier

[timmahrt]

This PR adds the mergeLabels() method to IntervalTier.

mergeLabels() can be used for example, to pull in the labels in a phone tier, into a word tier. So interval lists like:
[(0, 1, "hi")]
and
[(0, 0.5, "h"), (0.5, 1, "ai")]
would become
[(0, 1, "hi(h,ai)")]

[codecov]

Codecov Report

Base: 75.71% // Head: 75.81% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (dfd938c) compared to base (f57cba3).
Patch coverage: 93.75% of modified lines in pull request are covered.

Additional details and impacted files

@@              Coverage Diff               @@
##           praatio_v6      #47      +/-   ##
==============================================
+ Coverage       75.71%   75.81%   +0.10%     
==============================================
  Files              21       21              
  Lines            2623     2638      +15     
==============================================
+ Hits             1986     2000      +14     
- Misses            637      638       +1     

Impacted Files	Coverage Δ
praatio/data_classes/interval_tier.py	99.17% <93.75%> (-0.39%)	⬇️

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