Use block bootstrap for residual autocorrelation

[ncooder]

This PR replaces sample(res) in RSTRmodel with a block bootstrap. This helps to preserve the autocorrelation in residuals, since the current random shuffling destroys the time-series correlations. The block_bootstrap uses the optimal block length $n^{1/3}$ and maintains the same output format as previously.

[robjhyndman]

Thanks for the PR. But please move block_bootstrap code to avoid breaking the roxygen documentation for RSTRmodel.

[ncooder]

@robjhyndman The block_bootstrap has been moved.