Add mean logFC plots for DGE

[LuisHeinzlmeier]

Description of feature

Following up on #967 and #968, I also came across plotSmear() (edgeR) and plotMA() (DESeq2). Both basically plot the mean gene expression against the logFC. Let me know what you think! :)

Rdocumentation:

• plotSmear
• plotMA

[Zethson]

Let's do it!