[DEP] Deprecate BaseTransformer

[TonyBagnall]

last one out, turn off the lights, The whole lot will be removed, refactored or brushed under the carpet by release 0.11

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[MatthewMiddlehurst]

Could you run one of the currently deprecated estimators and make sure there are not 2 warning printed because of this?

[MatthewMiddlehurst]

I'm going to propose we just roll this into v1.0.0 with one single package-wide deprecation message. Helps us avoid the above issue, and by that point hopefully it's just on its own with nothing using it.