Deprecate wmedian and wquantile

[andreasnoack]

All the other weighted functions just use dispatch with an AbstractWeights vector.

[nalimilan]

We discussed this API with @ararslan some time ago. IIRC the conclusion was that weights should be passed via a weights keyword argument, so that you can also pass a plain Vector. But that requires moving the method to Statistics (which we should do anyway).

I guess we could still deprecate wmedian and wquantile in the meantime.

[ararslan]

I wholly support deprecating these functions. That said, it might be kind of confusing for users to migrate from wmedian to median with a weight positional argument then again to median with a keyword argument in a different package. If we're sure of the API we want in Statistics it might be a little less irritating for end users if we wait until we can make the changes there (unless of course that's way down the road).