User interface to `Scented` and `Unscented` Kalman inversions

## Constructors

The user interface to scented and unscented inversions defines two constructor-looking functions `UnscentedKalmanInversion` and `UnscentedKalmanInversionPostprocess`:

https://github.com/CliMA/OceanTurbulenceParameterEstimation.jl/blob/111e4c50fe33d46d9f7061e3256b9b16d70682c5/src/EnsembleKalmanInversions.jl#L204

https://github.com/CliMA/OceanTurbulenceParameterEstimation.jl/blob/111e4c50fe33d46d9f7061e3256b9b16d70682c5/src/EnsembleKalmanInversions.jl#L240

However, there are no objects / structs with names corresponding to those constructors.

Should we tweak the interface a little bit? It looks like there is a type called `Unscented` in `EnsembleKalmanProcesses.jl`:

https://github.com/CliMA/OceanTurbulenceParameterEstimation.jl/blob/111e4c50fe33d46d9f7061e3256b9b16d70682c5/src/EnsembleKalmanInversions.jl#L224

We could have users construct this object and pass to `EnsembleKalmanInversion`. It looks like the constructor for `EnsembleKalmanProcess` is slightly inconsistent for scented vs unscented inversion --- the scented flavor is

https://github.com/CliMA/OceanTurbulenceParameterEstimation.jl/blob/111e4c50fe33d46d9f7061e3256b9b16d70682c5/src/EnsembleKalmanInversions.jl#L155

We can still handle this.

## Post-processing

If we want to keep the post-processing function, I think we should call it something like `postprocess(eki::EnsembleKalmanInversion)`; then we can do the right thing whether `eki` represents a scented or unscented process.

However, another possibility is to give a kwarg to `iterate!`:

```julia
function iterate!(eki; iterations=1, postprocess=false)
```

when users select `postprocess=true`, we post-process results and store them inside the EKI object.

What do others think about these ideas? @adelinehillier @Zhengyu-Huang @navidcy

Start with the cited constructors and EnsembleKalmanProcess usage in src/EnsembleKalmanInversions.jl, then compare the Unscented type in EnsembleKalmanProcesses.jl. Review the open alternatives for constructing scented and unscented inversions and for post-processing. Done means the project has an agreed, consistent interface and the issue's proposed behavior is resolved.

Written by the indexing model from the issue text.