refs #335 Fix wrong ZSUM values calculation

UD(t) is included in the monetary mass M(t) so we can only use it on M(t-1) to calculate relative values.

Formulae for ZSUM values :

        t = last UD block
        t-1 = penultimate UD block
        M = Monetary mass
        N = Members count

        Q0  value = value - ( M(t-1) / N(t) )
        R0 value = (value / UD(t)) - (( M(t-1) / N(t) ) / UD(t))