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

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}
```

Start by reading the facet_grid behavior and the ggplot2 extending vignette linked in the issue. Define how an inherited facet_grid extension would accept a droplevels list for each factor and verify that selected levels are absent while the remaining facets and margin behavior are preserved.

Written by the indexing model from the issue text.