Replace interval numbers with attr names

[benjeffery]

I looked into replacing Interval with a dataclass, but we iterate on it a lot, so left it as is. I thought it worth keeping these changes from [0] to .left.

[codecov]

Codecov Report

Merging #1264 (ba708bd) into main (40159b6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1264      +/-   ##
==========================================
- Coverage   93.77%   93.77%   -0.01%     
==========================================
  Files          26       26              
  Lines       21940    21939       -1     
  Branches      971      971              
==========================================
- Hits        20574    20573       -1     
  Misses       1325     1325              
  Partials       41       41              

Flag	Coverage Δ
c-tests	92.49% <ø> (ø)
lwt-tests	92.97% <ø> (ø)
python-c-tests	95.01% <100.00%> (-0.01%)	⬇️
python-tests	98.66% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
python/tskit/cli.py	96.26% <100.00%> (-0.03%)	⬇️
python/tskit/combinatorics.py	99.20% <100.00%> (ø)
python/tskit/trees.py	97.44% <100.00%> (ø)

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[jeromekelleher]

Nice improvement to readability, thanks.

We definitely do need to keep the tuple interpretation though.