Move `\Delta t`to be stored as a "Process" variable

[odunbar]

Unless we have a suitably general way of including timestepping into every EKP, then this object really should be a process variable of Inversion and Sampler, and not an EnsembleKalmanProcess variable

@bielim co-authored

[ilopezgp]

I believe we always do have a way to include the time step? It comes from the discretization of the continuous form of the updates, and we can do the same for UKI, as shown in Huang et al, 2021.

[odunbar]

We create an array in all cases. It should be noted that in (Sparse) EKI and EKS have an array DeltaT with DeltaT[current_step] being the current Delta t. Currently i believe UKI has a single entry array DeltaT = [Delta t] that is not updated?

[ilopezgp]

@odunbar see #109 for a fix.