LQ name conflict

Now that Julia has an LQ type (in the LinearAlgebra stdlib), there's a name conflict with the type exported by the package.

One way to handle this would be to change the following to ... struct LQProblem with the appropriate constructor.

mutable struct LQ
    Q::ScalarOrArray
    R::ScalarOrArray
    A::ScalarOrArray
    B::ScalarOrArray
    C::ScalarOrArray
    N::ScalarOrArray
    bet::Real
    capT::Union{Int, Nothing} # terminal period
    rf::ScalarOrArray
    P::ScalarOrArray
    d::Real
    F::ScalarOrArray # policy rule
end

https://github.com/QuantEcon/lecture-source-jl/issues/436