[FEATURE] Support initial configuration list() in current session?

[kevinrue]

Thinking of the case where the list(<Panel>) for a particular dataset is already constructed in the same R session, and the user just wants to use that directly rather than saving it to file.

Context

This is not too unusual, especially in cases where the user might not control the underlying CSV file (so they can't easily change the paths, or the per-dataset configuration files), e.g., for a shared R installation on a HPC, or even on a prebuilt Docker image.

In such cases, the user could read in the initial CSV and replace certain elements of uri with a modified list(<Panel>) directly in their app.R. Users could also dump it to an RDS file but this may be relatively expensive compared to handling it directly in memory.

This may not be incompatible with the current design, given that uri is fundamentally about getting the per-dataset list(<Panel>) anyway; it would just generalise it so that users aren't forced to the filesystem.

[kevinrue]

document/demonstrate how to do it in a vignette