[ENH] Rework _HeterogenousMetaEstimator and composable estimators

[MatthewMiddlehurst]

This is a bit big and messy, but a lot of these composable estimators are intertwined with the mixin which is to be expected really. Just went for a tidy of the whole lot.

• renames _HeterogenousMetaEstimator to _ComposableEstimatorMixin
• reworks the class, removing a lot of the methods and functions. Centres input validation around two methods, _check_estimators and _convert_estimators
• the methods for getting and setting params/attributes should work as intended like the base class now
• remade ClassifierChannelEnsemble to work with the changed mixin.
• removes the deprecated WeightedEnsembleClassifier class
• Adds tests to verify that expected results arrays are actually being used

part of #1518

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I have added the following labels to this PR based on the title: [ $\color{#FEF1BE}{\textsf{enhancement}}$ ].
I have added the following labels to this PR based on the changes made: [ $\color{#BCAE15}{\textsf{classification}}$, $\color{#7E0206}{\textsf{regression}}$, $\color{#2C2F20}{\textsf{testing}}$ ]. Feel free to change these if they do not properly represent the PR.

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[TonyBagnall]

have you checked coverage on the related classes? It was previously low

[MatthewMiddlehurst]

cant check currently