Removing `BaseKernel`

[theogf]

I am not sure what changed, but now BaseKernel has no reason to exist apparently. Doing a search on where it is used shows it clearly. I think we can also drop these deprecated methods:

KernelFunctions.jl/src/generic.jl

Lines 14 to 18 in 86cde32

	for k in nameof.(subtypes(BaseKernel))
	@eval begin
	@deprecate($k(ρ::Real;args...),transform($k(args...),ρ))
	@deprecate($k(ρ::AbstractVector{<:Real};args...),transform($k(args...),ρ))
	end

The only question is : Are we replacing SimpleKernel by BaseKernel for kernels using Distances.jl or are we keeping SimpleKernel and simply remove BaseKernel ?

[willtebbutt]

I can't see a reason to keep BaseKernel.

[theogf]

Solved by #150