Interface of KernelDensity.pdf is not consistent with Distributions.pdf

[jaksle]

I've noticed KernelDensity.pdf cannot be used in the same way as Distributions.pdf. I think this is very confusing for users and is a potential source of errors. In particular, pdf.(kde::UnivariateKDE, x::Vector) and pdf(kde::BivariateKDE, M::Matrix) do not work.

I could try adding them if there is agreement it should be done.