gg_add_generation_bands: Add bands of transparent colors to denote generations on...
In CKMRpop: Forward-in-Time Simulation and Tallying of Pairwise Relationships

Pass it the original ggplot, and this will return it, with the bands added

gg_add_generation_bands(
  g,
  L,
  alpha = 0.2,
  colors = c("red", "orange", "yellow", "green", "blue"),
  add_impossibles = FALSE
)

`g`	the original ggplot
`L`	the number of rows (or columns) in the ancestor-match matrices
`alpha`	the transparency to use for these color-bands
`colors`	the colors in order of self, parent, grandparent, etc. By default it is just rainbow order starting from red.
`add_impossibles`	pass TRUE if you want to blot out the cells that are not possible because they conflict with the sex of the individuals. You set the fill of the impossibles with a `scale_fill_manual()` in the main ggplot call. i.e., scale_fill_manual(values = c(`FALSE` = NA, Impossible = "white", `TRUE` = "black"))