Add module for PCA

[Gregor Sturm]

Add a module that generates a 2-dimensional PCA plot of the samples.

Input:

• gene expression matrix
• sample sheet

Ouput:

• plot

Section in old code:
https://github.com/icbi-lab/deseq2icbi/blob/28e54a75598d39b70090376daa0e9ddefee25363/runDESeq2_ICBI.R#L481-L491

[Gregor Sturm]

Closed via #25