Added: ABC for stochastic volatility model

Noisy AR1.R: Fixed an error in the code.
SV AR1.R: The IS and rej ABC are tested for the stochatic volatility
model with AR1 volatility. True parameters are taken from Sandmann and
Koopman (1998) corresponding to case Psi_4 in its Table 2.

Results 1: The estimating accuracy of phi and alpha is comparable to the
literature, while that of sigma is very bad. See the comments in the
code.
Results 2: The estimated mean and sd in the pilot step don't change
much, making the IS performs similarly to rej. A better adaptive
algorithm is needed.