Provide proper depreciation to EuclideanClusterComparator

[SergioRAgostinho]

Following up on the discussion of #1542 (comment)

[taketwo]

I am not aware of any proper way to deprecate the removed template parameter, so I think we will break API anyway.

But of course, It would be nice to add depreciation warnings to removed functions. The user will see a compiler error (because of mismatched template list) and these warnings from follow-up function calls. So perhaps it would be handy to put a note about what happened with template parameters in these warnings.

[SergioRAgostinho]

we can leave the template parameter there and remove it in the future.

[taketwo]

True. So when users use deprecated functions they will be warned both about the functions themselves and about the future template parameter removal. Sounds good for me.