Concat, Append and Add methods

[JoFrhwld]

This implements a number of concat, append and adding methods for the purpose of building AlignedTextGrids programmatically.

At the moment, this is still a draft.

[codecov]

Codecov Report

Attention: Patch coverage is 94.72097% with 35 lines in your changes missing coverage. Please review.

Project coverage is 91.33%. Comparing base (0cc7eb1) to head (079cef2).
Report is 1 commits behind head on dev.

Files	Patch %	Lines
src/aligned_textgrid/mixins/tiermixins.py	91.58%	9 Missing ⚠️
src/aligned_textgrid/sequences/sequences.py	93.52%	9 Missing ⚠️
src/aligned_textgrid/sequence_list.py	94.07%	8 Missing ⚠️
src/aligned_textgrid/aligned_textgrid.py	95.08%	3 Missing ⚠️
src/aligned_textgrid/mixins/mixins.py	95.00%	2 Missing ⚠️
src/aligned_textgrid/mixins/within.py	92.30%	1 Missing ⚠️
src/aligned_textgrid/points/points.py	96.55%	1 Missing ⚠️
src/aligned_textgrid/points/tiers.py	95.65%	1 Missing ⚠️
src/aligned_textgrid/sequences/tiers.py	99.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #193      +/-   ##
==========================================
+ Coverage   90.70%   91.33%   +0.62%     
==========================================
  Files          14       15       +1     
  Lines        1431     1927     +496     
==========================================
+ Hits         1298     1760     +462     
- Misses        133      167      +34     

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