Latent Variable Support for Analysis Summed

[Jammy2211]

The following PR implements a hotfix to get latent variables working for summed Analysis objects:

#1084

We need to flesh out the PR so that latent variables fully use all summed Analysis features, for example when the model has free parameters that vary across the analysis objects.

However, it may just be that no source code changes are required and its up to a user to customize their compute_latent_variables correctly.

We can discuss this in the new year.