Zarray: Create forecast design matrices
In gamalamboy/stresstest: Implement the ETI Stress Testing Simulation

This performs the workhorse creation of an array with design matrices for use in the forecasting exercises. It creates an RcppArmadillo cube with each slice being a design matrix whose lagged values are updated with the Bayesian posterior predictive distribution's sampled values.

1	Zarray(datindex, datarows, Zindex, Zmat)

`datindex`	a linear index mapping the data columns to the model structure. It is generally a product of `sub2inds()`.
`datarows`	the data rows used for creating the design matrix, of size <lag length of VAR(p)> + 1.
`Zindex`	a linear index which maps the non-zero entries in the new design matrix.
`Zmat`	a fixed design matrix template.