Reorganise the next item rules into subdirectories

[frankier]

No description provided.

[github-actions]

Benchmark Results

	main	0a089ef...	main/0a089ef35c2e85...
next_item_rules/4pl/mean_point_mepv_10	2.56 ± 0.014 ms	2.56 ± 0.014 ms	1
next_item_rules/4pl/mean_point_mepv_bare	0.876 ± 0.0075 ms	0.864 ± 0.01 ms	1.01
next_item_rules/4pl/mode_point_mepv_10	2.02 ± 0.012 ms	2.02 ± 0.011 ms	1
next_item_rules/4pl/mode_point_mepv_bare	0.814 ± 0.0081 ms	0.819 ± 0.0076 ms	0.993
time_to_load	1.63 ± 0.007 s	1.63 ± 0.0056 s	1

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).