Implement data set builders and the new entity/relationship model

[mdekstrand]

This adds a more thorough data model to LensKit, along with dataset builders to build more advanced data sets.

Not every feature is implemented yet, but it should be enough for the current code to keep working.

Closes #547. Closes #586. Closes #587.

[codecov]

Codecov Report

Attention: Patch coverage is 94.76615% with 47 lines in your changes missing coverage. Please review.

Project coverage is 90.32%. Comparing base (01907a8) to head (24046eb).
Report is 80 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/data/dataset.py	93.96%	23 Missing ⚠️
lenskit/lenskit/data/builder.py	95.47%	12 Missing ⚠️
lenskit/lenskit/data/schema.py	90.78%	7 Missing ⚠️
lenskit/lenskit/data/container.py	95.55%	2 Missing ⚠️
lenskit/lenskit/data/mtarray.py	75.00%	1 Missing ⚠️
lenskit/lenskit/data/vocab.py	0.00%	1 Missing ⚠️
lenskit/lenskit/splitting/split.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #610      +/-   ##
==========================================
+ Coverage   90.04%   90.32%   +0.27%     
==========================================
  Files         108      109       +1     
  Lines        6814     7269     +455     
==========================================
+ Hits         6136     6566     +430     
- Misses        678      703      +25     

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