plot_boxplots returns nothing if not set to return figure

[shihsama]

Hi @johannesostner ,
So glad to see this integrative work! I'm an old user of scCODA. Hence, I noticed a change in boxplot function. In the old scCODA, if we set plot_facets = True, then it would return a FacetGrid object so that we can modify specific subplot. However, in pertpy, the plot_boxplots function does not return FacetGrid if we set plot_facets = True and return_fig= False. It actually returns NoneType. So I checked the differences between the source codes of pertpy and scCODA.
I found plot_boxplots would not return FacetGrid in the end even if we set plot_facets = False. I believe here is the rub.
Another issue is that the argument show = True is somewhat redundant. After removing it, we are able to modify titles of subplots.

The original codes in the end:

    if show:
        plt.show()
    if return_fig:
        return plt.gcf()
    return None

Codes modified:

    if return_fig:
        return plt.gcf()
    else:
        return g
    return None

###The arguement show = True should also be removed.

In this way, it finally returns <seaborn.axisgrid.FacetGrid> when we set plot_facets = False and plot_facets = True.
Thanks for reviewing this!