Dev:filter - Filter implementation  PROPOSED FOR 4.0

The filter directive is implemented using an augmented transform object. The filter is converted to a discrete-time state-space model using the canonical observable form for a strictly causal system:

H(z) = \frac {\sum_{n=1}^{N-1} b_n z^{n-N}} {z^N + \sum_{n=1}^{N-1} a_n z^{n-N}} \to \left\{ \begin{aligned} \dot x &= A x + B u \\ y &= C x \end{aligned} \right.

where

$A = \begin{bmatrix} 0 & 0 & \cdots & 0 & -a_n \\ 1 & 0 & \cdots & 0 & -a_{n-1} \\ \vdots & \ddots & \ddots & \vdots & \vdots \\ 0 & 0 & \ddots & 0 & -a_2 \\ 0 & 0 & \cdots & 1 & -a_1 \end{bmatrix} \quad B = \begin{bmatrix} b_n \\ b_{n-1} \\ \vdots \\ b_2 \\ b_1 \end{bmatrix} \quad C = \begin{bmatrix} 0 & 0 & \cdots & 0 & 1 \end{bmatrix}$

The state vector x is update whenever the global clock modulo the time step is equal to the time skew.