Add ensemble learning to package

[NickT42]

This PR is meant to add support for the build_ensemble function, which has been moved from the pull request in 'ModalDecisionLists.jl' to here as it's more fitting to the 'SoleModels.jl' package.
The other changes include

1. The addition of missing functions such as atoms() were added to support alphabet() for DecisionLists
2. The addition of a custom apply() function for DecisionEnsembles to (optionally) handle different submodels accessing only certain subsets of the features in the data.

Note that the slicing in the new apply function currently risks causing issues if the tabulardataset in the propositional logiset on which the ensemble is to be applied does not support all '__getindex()' features, but that is a problem to be solved by having stricter prerequisites for the table in 'PropositionalLogiset'.

[codecov]

Codecov Report

❌ Patch coverage is 22.64151% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.62%. Comparing base (c1b3f36) to head (0a7539c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/utils/models/ensemble_learning.jl	0.00%	32 Missing ⚠️
src/utils/models/syntax-utilities.jl	0.00%	6 Missing ⚠️
src/utils/models/ensembles.jl	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
- Coverage   56.37%   54.62%   -1.76%     
==========================================
  Files          20       21       +1     
  Lines        1254     1307      +53     
==========================================
+ Hits          707      714       +7     
- Misses        547      593      +46     

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