[DOC,ENH] base docs and testing

[MatthewMiddlehurst]

Testing and documentation for base classes. Refactors a few usages of get_class_tag where it is unnecessary and a couple of other minor refactors which explode the file change count.

[aeon-actions-bot]

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I have added the following labels to this PR based on the title: [ $\color{#F3B9F8}{\textsf{documentation}}$, $\color{#FEF1BE}{\textsf{enhancement}}$ ].

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[TonyBagnall]

LGTM