[ENH] Collection and series base tidy

[MatthewMiddlehurst]

• makes __init__ abstracts for the classes in base
• renames the base series data arrays to contain series in the var name
• check for soft dependencies in BaseAeonEstimator rather than in the base classes below
• remove self.fit_time_ from some collection estimators and the collection base. This was not very consistently used.
• tidy up docs a bit

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[TonyBagnall]

failing tests?