Handling of case column in linelist data class

[athowes]

Following PR #390, at the moment as_epidist_linelist does not require there to be a case ID column.

At the moment, the models in the package do not use a case column.

That said, a case column would be useful for plotting functionality.

Our options:

1. Enforce users providing a case column
2. Don't expect a case column and don't create one
3. Create a case column if not provided one (e.g. in the order of the data)