Add function to unpack state to 3D array(s)

[briochemc]

Right now I do this via

const iDIP = 1:nb
const iDOP = iDIP .+ nb
const iPOP = iDOP .+ nb
function unpack_state(x)
DIP = x[iDIP]
DOP = x[iDOP]
POP = x[iPOP]
return DIP, DOP, POP
end

and

DIP, DOP, POP = unpack_state(x)

and

DIP_3d = NaN * wet3d; DIP_3d[iwet] = DIP

Probably good to have a unpack_state and vector_to_3D functions in AIBECS