Measurement of the ZZ production cross section in proton-proton collisions at s = 8 $$\sqrts=8$$ TeV using the ZZ → ℓ−ℓ+ℓ′−ℓ′+ and Z Z → ℓ − ℓ + ν ν ¯ $$ZZ\to \ell^-\ell^+\nu \overline\nu$$ decay channels with the ATLAS detector