AlignedTextGrid creation flexibilization

[JoFrhwld]

A new AlignedTextGrid can now be created with a list of TierGroups

atg = AlignedTextGrid([TierGroup(), TierGroup()])

The way class-cloning is done has also been streamlined so that atg.append() will appropriately clone the classes of a tier group, re-using already cloned classes if they exist.

[codecov]

Codecov Report

Attention: Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.32%. Comparing base (079cef2) to head (2c8a77f).
Report is 11 commits behind head on dev.

Files	Patch %	Lines
src/aligned_textgrid/aligned_textgrid.py	95.55%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #199      +/-   ##
==========================================
- Coverage   91.33%   91.32%   -0.02%     
==========================================
  Files          15       15              
  Lines        1927     1936       +9     
==========================================
+ Hits         1760     1768       +8     
- Misses        167      168       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.