Define observation/control alignment for discrete-time models

[MatthieuDarcy]

Add observation_control_alignment for discrete-time Simulator (#312).

Mathematical summary

Implements the same_time vs previous time distinction. same_time (default, preserves the existing behavior) implements:

$$ \begin{aligned} x_0 &\sim p_0 \\ y_0 &\sim p(\cdot |x_0, u_0) \end{aligned} $$

and subsequently

$$ \begin{aligned} x_{k+1} &\sim p(\cdot | x_k, u_k)\\ y_k &\sim p(\cdot| x_k, u_k) \end{aligned} $$

previous_time implements

$$ \begin{aligned} x_0 &\sim p_{0}\\ x_{k+1} &\sim p(\cdot | x_k, u_k) \\ y_{k+1} &\sim p(\cdot| x_{k+1}, u_k) \end{aligned} $$

Summary of changes

Adds an explicit property of DynamicalModel defined at initialization called observation_control_alignment: Literal[ "same_time", "previous_transition" ] = "same_time"

When defined as previous_transition, this results in the following behavior

1. $y_0$ cannot be sampled. Hence observations $y$ is of size $T-1$ while the states $x$ are of size $T$.
2. I added the controls $u$ in the returnedSimulatedResults. This is very practical when doing MPC. This leads to a similar behavior as observations: when using "same_time" it is of size $T$, when using "previous_time" it is of size $T-1$.

This means that states and times are of different sizes to controls and observations (specifically size $T$ vs $T-1$) when using previous_transition.

1. MPPI is updated to use this convention in Fixed MPPI #348

Still to be done

1. previous_transition only works for simulation, no conditioning and not filtering. Worth delegating to a seperate PR?