Remove hard-coded `Vector` fields and add vectors constructors

Right now kernels like LinearKernel have two problems: they can only take Real arguments (no way to pass kernel.c) as an argument and also they do not allow for different AbstractVector types to be stored.
The interested use case is when using GPUs, one cannot do kernelmatrix on CuArrays without this.

struct LinearKernel{Tc<:Real} <: SimpleKernel
    c::Vector{Tc}

    function LinearKernel(c::Real)
        @check_args(LinearKernel, c, c >= zero(c), "c ≥ 0")
        return new{typeof(c)}([c])
    end
end

should be

struct LinearKernel{Tc<:Real,Vc<:AbstractVector{<:Tc}} <: SimpleKernel
    c::Vc
    function LinearKernel(c::V) where {T<:Real, V<:AbstractVector{T}}
        @check_args(LinearKernel, first(c), first(c) >= zero(c), "c ≥ 0")
        return new{T,V}(c)
    end
    function LinearKernel(c::Real)
        @check_args(LinearKernel, c, c >= zero(c), "c ≥ 0")
        C = [c]
        return new{eltype(C),typeof(C)}(C)
    end
end

Start with the LinearKernel definition shown in the issue, then locate the other kernel structs that store hard-coded Vector fields and their constructors. Check how vector-valued parameters are used with GPU arrays, and confirm that the affected kernels accept AbstractVector types while scalar construction still works. Done means the relevant constructors and field types support the intended vector types without breaking existing scalar behavior.