Try simple extension of `facet_grid` to remove specific levels of factor from plot

[cicdguy]

Motivation: If we want to compare the BEP vs. All population using the margin = TRUE in facet_grid() works. However, it also gives us the non-BEP facets. We might not need/want to show this.

Idea: We could inherit from facet_grid and add additional argument droplevels which could be a list with one element per factor used, which can specify which levels not to have in the final plot. See https://cran.r-project.org/web/packages/ggplot2/vignettes/extending-ggplot2.html

Provenance:

Creator: Sabanes Bove, Daniel {MDBR~Basel}