A reppop table in 'gscramble' is used to define how the founder populations
in a GSP (typically named something like "A", "B", etc.) are mapped to the
groups/populations of individuals, as specified in the individual meta data
(an example of which is found in I_meta).
A tibble with three columns: index, which must be of type integer,
pop, and group of type character.
The developers created this.
This particular version shows a situation where individuals from four different
groups (Pop1, Pop2, Pop3, and Pop4) get mapped to four different founder groups
(A, B, C, D) in the GSP.
Since this RepPop4 example has three values of index: 1, 2, and 3,
it specifies that there will be three rounds of sampling of individuals
from the populations to be the founders on the pedigree. That will be done
entirely without replacement (individuals are not replaced after each round!)