Correct YoY Rate

Function Correct YoY Rate within Seasonalityreturns the corrected YoY rate r' for a given YoY rate r, date t and an inflation curve.
If the curve has Interp Index = FALSE, the date t is shifted to the begining of the month where the index is defined.
In the case of Multiplicative the calculation formula is:
r' = (r + 1)f - 1
where
f = F(t)/F(t-1year)
where
F(t) is the seasonality factor as of time t as returned by the function Factor

In the case of Kerkhof this function is not implemented.