Implement envelope tracker

[austin-hoover]

This PR implements an envelope/centroid tracker. We track the covariance matrix $\mathbf{\Sigma} = \langle \mathbf{x} \mathbf{x}^T \rangle$ and centroid $\mathbf{\mu} = \langle \mathbf{x} \rangle$, where $\mathbf{x} = [x, x', y, y', z, \Delta E ]^T$ is the 6D phase space vector.

Evolution equations:

$$ \begin{align} \mathbf{x} &\rightarrow \mathbf{M} \mathbf{x} + \mathbf{u} , \\ \mathbf{\mu} &\rightarrow \mathbf{M} \mathbf{\mu} + \mathbf{u} , \\ \mathbf{\Sigma} &\rightarrow \mathbf{M} \mathbf{\Sigma} \mathbf{M}^T . \end{align} $$

We track the $7 \times 7$ matrix $\mathbf{S} = \langle \mathbf{y} \mathbf{y}^T \rangle$, where $\mathbf{y} = [x, x', y, y', z, dE, 1]^T$:

$$ \mathbf{S} = \langle \mathbf{y} \mathbf{y}^T \rangle = \begin{bmatrix} \langle \mathbf{x} \mathbf{x}^T \rangle & \langle \mathbf{x} \rangle \\ \langle \mathbf{x} \rangle^T & 1 \end{bmatrix} = \begin{bmatrix} \mathbf{\Sigma} + \mathbf{\mu}\mathbf{\mu}^T & \mathbf{\mu} \\ \mathbf{\mu}^T & 1 \end{bmatrix} . $$

Evolution equations for $\mathbf{y}$:

$$ \begin{align} \mathbf{y} &\rightarrow \mathbf{N} \mathbf{y} , \\ \mathbf{S} &\rightarrow \mathbf{N} \mathbf{S} \mathbf{N}^T , \end{align} $$

where $\mathbf{N}$ is defined as

$$ \mathbf{N} = \begin{bmatrix} \mathbf{M} & \mathbf{u} \\ \mathbf{0} & 1 \end{bmatrix} . $$

Accelerator nodes and child nodes are mapped to transfer matrices at the Python level. An error is raised if the node is not recognized. Space charge is handled by assuming a uniform charge density within an ellipsoid in the $x$-$y$-$z$ plane (for 3D space charge) or within an ellipse in the $x$-$y$ plane (for 2D space charge).