rename imputer classes

[solegalli]

• MeanMedianImputer into MeanImputer
• AddMissingIndicator into MissingIndicator
• NumericalAbitraryImputer into ArbitraryImputer

We use 1 cycle of deprecation warnings, so old classes should still remain with a future warning that is raised at init saying that this class was deprecated in version 2.0.0 in favour of... and will be removed in version 2.1.0. User should use [new name of class] instead.

Add #TODO remove in version 2.1.0 on top of deprecated class.

tests should pass for both classes in version 2,0,0

[solegalli]

@favourone is this one one for you as well? :)

[ojassharma7]

Hi! I'd like to work on this issue — A verified fix is ready; opening a PR after local review.

I'll open a pull request shortly. Please let me know if it's already taken.

[aniq63]

Following the review feedback, the work has now been split into separate PRs:

• Rename MeanMedianImputer to MeanImputer #975 – Rename MeanMedianImputer to MeanImputer (by @ojassharma7)
• Rename AddMissingIndicator to MissingIndicator with deprecation support #976 – Rename AddMissingIndicator to MissingIndicator with deprecation support
• Rename ArbitraryNumberImputer to ArbitraryImputer with deprecation support #977 – Rename ArbitraryNumberImputer to ArbitraryImputer with deprecation support

These PRs include the requested code, documentation, file renames, and test updates.