Bug fix in the definition of the PAC expectation (MCE version).

Replaced

         ᵐ                              ᵐ⁻¹ ᵐ⁻¹
  Zₜ = - ∑  𝛼ᵢ 𝛽ⁱ⁺¹ Zₜ₊ᵢ + A(1) [ 𝛥 yₜ + ∑   ∑   𝛼ⱼ₊₁𝛽ʲ⁺¹𝛥 yₜ₊ₖ ]
        ᵢ₌₁                             ₖ₌₁ ⱼ₌ₖ

by

         ᵐ                              ᵐ⁻¹ ᵐ⁻¹
  Zₜ = - ∑  𝛼ᵢ 𝛽ⁱ⁺¹ Zₜ₊ᵢ + A(1) [ 𝛥 yₜ - ∑   ∑   𝛼ⱼ₊₁𝛽ʲ⁺¹𝛥 yₜ₊ₖ ]
        ᵢ₌₁                             ₖ₌₁ ⱼ₌ₖ

Also:

 - Added comments and comparison with the formula in Brayton et alii (2000),
 - Removed unused input argument,
 - Removed unused variables.

(cherry picked from commit 798b5601)