[NDTensors] Introduce LabelledNumbers and GradedAxesNext

[mtfishman]

This is a rewrite and redesign of GradedAxes, for now called GradedAxesNext but the goal will be to remove GradedAxes and replace it with the code in GradedAxesNext (either here or in a follow-up PR). The goal is to make it easier to implement slicing operations of graded unit ranges which preserve the sector labels (which is needed for finishing #1332), which I have found is a bit more cumbersome than I think is necessary with the current GradedAxes design.

@emstoudenmire @ogauthe

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.78%. Comparing base (638624f) to head (887f612).

❗ Current head 887f612 differs from pull request most recent head 97cbccd. Consider uploading reports for the commit 97cbccd to get more accurate results

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1351       +/-   ##
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- Coverage   84.40%   53.78%   -30.62%     
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  Files         100       99        -1     
  Lines        8581     8528       -53     
===========================================
- Hits         7243     4587     -2656     
- Misses       1338     3941     +2603     

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[mtfishman]

@ogauthe this doesn't have all of the features of GradedAxes, but it has a lot more support for slicing (see the tests). I will merge this as-is and we can start adding more features to it, like fusion, and plan to get ride of GradedAxes as soon as possible.

[ogauthe]

Great. I paused my work on GradedAxes to wait for your updates.