Add example data management

[billdenney]

Here is an example of how I would create typical data management for an nlmixr2 dataset.

To take it from here to NONMEM, I would:

1. Convert the ID column to numeric with: ID = as.integer(factor(USUBJID))
2. Convert the CMT column to numeric with a data merge (preferred) or a case_when() (okay).
3. Sort the column order so that numeric columns came first (something like select(where(is.numeric), everything()))