Add dataset-based data splitting code

[mdekstrand]

This moves (well, copies for now — need to update usages) the data splitting code into lenskit.splitting and makes it work on Dataset instead of data frames.

[github-actions]

The GitHub 🤖 has run the tests on your PR.

Covered 100.00% of diff (coverage changed 0.01% from 92.11% to 92.12%).

origin/main...HEAD, staged and unstaged changes

• lenskit/lenskit/data/dataset.py (100%)
• lenskit/lenskit/data/items.py (75.8%): Missing lines 190,370-373,375,403,440
• lenskit/lenskit/data/vocab.py (100%)
• lenskit/lenskit/splitting/init.py (100%)
• lenskit/lenskit/splitting/holdout.py (92.9%): Missing lines 39,106,110,135
• lenskit/lenskit/splitting/records.py (100%)
• lenskit/lenskit/splitting/split.py (77.8%): Missing lines 45,52,60,64-66
• lenskit/lenskit/splitting/users.py (100%)

Summary

• Total: 242 lines
• Missing: 18 lines
• Coverage: 92%

Source Coverage Report

Name	Stmts	Miss	Cover
lenskit-funksvd/lenskit/funksvd.py	187	8	96%
lenskit-hpf/lenskit/hpf.py	24	0	100%
lenskit-implicit/lenskit/implicit.py	94	9	90%
lenskit/lenskit/algorithms/__init__.py	67	8	88%
lenskit/lenskit/algorithms/als/__init__.py	3	0	100%
lenskit/lenskit/algorithms/als/common.py	129	2	98%
lenskit/lenskit/algorithms/als/explicit.py	121	3	98%
lenskit/lenskit/algorithms/als/implicit.py	112	1	99%
lenskit/lenskit/algorithms/basic.py	161	4	98%
lenskit/lenskit/algorithms/bias.py	150	3	98%
lenskit/lenskit/algorithms/knn/__init__.py	3	0	100%
lenskit/lenskit/algorithms/knn/item.py	303	17	94%
lenskit/lenskit/algorithms/knn/user.py	178	11	94%
lenskit/lenskit/algorithms/mf_common.py	61	0	100%
lenskit/lenskit/algorithms/ranking.py	75	11	85%
lenskit/lenskit/algorithms/svd.py	75	4	95%
lenskit/lenskit/batch/__init__.py	2	0	100%
lenskit/lenskit/batch/_predict.py	30	2	93%
lenskit/lenskit/batch/_recommend.py	46	4	91%
lenskit/lenskit/crossfold.py	136	2	99%
lenskit/lenskit/data/__init__.py	8	0	100%
lenskit/lenskit/data/checks.py	37	0	100%
lenskit/lenskit/data/dataset.py	364	19	95%
lenskit/lenskit/data/fetch.py	38	28	26%
lenskit/lenskit/data/items.py	176	11	94%
lenskit/lenskit/data/matrix.py	115	5	96%
lenskit/lenskit/data/movielens.py	96	18	81%
lenskit/lenskit/data/mtarray.py	57	3	95%
lenskit/lenskit/data/tables.py	25	0	100%
lenskit/lenskit/data/vocab.py	90	7	92%
lenskit/lenskit/diagnostics.py	4	0	100%
lenskit/lenskit/math/__init__.py	0	0	100%
lenskit/lenskit/math/solve.py	6	0	100%
lenskit/lenskit/metrics/__init__.py	0	0	100%
lenskit/lenskit/metrics/predict.py	32	0	100%
lenskit/lenskit/metrics/topn.py	212	1	99%
lenskit/lenskit/parallel/__init__.py	4	0	100%
lenskit/lenskit/parallel/chunking.py	20	1	95%
lenskit/lenskit/parallel/config.py	65	8	88%
lenskit/lenskit/parallel/invoker.py	31	2	94%
lenskit/lenskit/parallel/pool.py	54	9	83%
lenskit/lenskit/parallel/sequential.py	22	0	100%
lenskit/lenskit/parallel/serialize.py	51	1	98%
lenskit/lenskit/parallel/worker.py	43	3	93%
lenskit/lenskit/splitting/__init__.py	4	0	100%
lenskit/lenskit/splitting/holdout.py	56	4	93%
lenskit/lenskit/splitting/records.py	55	0	100%
lenskit/lenskit/splitting/split.py	27	6	78%
lenskit/lenskit/splitting/users.py	60	0	100%
lenskit/lenskit/topn.py	109	25	77%
lenskit/lenskit/util/__init__.py	72	19	74%
lenskit/lenskit/util/envcheck.py	57	44	23%
lenskit/lenskit/util/logging.py	19	0	100%
lenskit/lenskit/util/random.py	26	3	88%
lenskit/lenskit/util/test.py	103	19	82%
lenskit/lenskit/util/timing.py	28	0	100%
TOTAL	4123	325	92%