Extend API to get specific parameters/outputs

## Issues

For structured parameters objects constructed by combining several `ParameterDistributions` or `Observation` and minibatched with `ObservationSeries`, users have an opportunity to provide a name with each of these items. We should to provide an API to be able to extract these by name, e.g. for parameters and observations
- `"param_1","param_2"` -> used to get specific parameter dist dimensions
- `"obs_1", "obs_2", "obs_3"` -> used to get specific obs within an observation series
- `"series_1"`, `"series_2"`, -> [minibatching only] used in place of "iteration" to get iterations corresponding to minibatch where series item is used.

Examples of how this may look:
```julia
get_u(ekp, iteration, "param_2") # dimensions corresponding to parameter 2 at iteration
get_g(ekp, "obs_1") # obs_1 over all iterations
get_obs(ekp, iteration, ["obs_2", "obs_3"]) # obs_2 and obs_3 for iteration
get_obs_noise_cov(ekp, "series_2", "obs_2") # obs_2 covariances from the observation-series item series_2
```

Trace the existing get_u, get_g, get_obs, and get_obs_noise_cov entry points, along with the ParameterDistributions, Observation, and ObservationSeries construction paths. Determine how names and minibatch series are represented, then verify that the requested named parameter, observation, and series lookups work consistently across the example cases.

Written by the indexing model from the issue text.