Set up initial problem statement in the vignette

[kaitejohnson]

Goal
Demonstrate an example nowcasting problem, starting initially with a long form data frame with counts of new confirmations indexed by reference date and report date (similar to epinowcast vignette).

Solution
Load in the example data (we could make this package data, but to start we could just use epinowcast's package data).

Describe alternatives you've considered
Other options:

• start from a different public data source e.g. https://github.com/kassteele/Nowcasting/blob/master/data/measles_NL_2013_2014.dat where we could then show how to convert from line list at a visit level data to a long tidy data frame of counts by reference and report date
• create synthetic nowcast data with a known reporting delay and uncertainty, and format it however we want the starting point of the workflow to be.

Additional context

• We would like baselinenowcast to be compatible with and use the pre and post-processing in epinowcast. Exactly what the required inputs for baselinenowcast are is still being determined.