overload 3 arg * to perform quad, and X_A_Xt

[oxinabox]

quad(a, x)          # compute x' * a * x when `x` is a vector.
X_A_Xt(a, x)        # compute `x * a * x'` for a matrix `x`.

Right now, to take advantage of quad and X_A_Xt the user must ask for them directly.
Which is annoying because the user might not know they are dealing with a PDMat.
If the user wrote x' * a * x then it would not hit these methods.

But we could fix this via something like:

function Base.(:*)(x::Union{Adjoint{<:AbstractVector}, Transpose{<:AbstractVector},  a::AbstractPDMat, y::AbstractVector)
    if partent(x) == y
        quad(a, x)
    else
        invoke(...)  # call the generic `*`
    end
end

And similar for X_A_Xt