PCA analysis on genes and transcripts TPM for kallisto and salmon

• Delete unused files scripts/fg_extract_transcripts.py, scripts/heatmap_and_clustermap.py, scripts/perform_PCA.py.
• Add rules (pca_kallisto, pca_salmon) that use zpca on genes and transcripts TPM tables from kallisto and salmon.
• The output is wired to multiqc_report but the plots are not visualized to multiqc.
• Update documentation.
• Update dag and rulegraph.
• Note: No tests are added, because of random numbers used. No fixed md5sum.

Fixes #140 (closed) #142 (closed)

Before merging I would ask:

• @boersch check if the current PCA analysis gives similar results to the standard PCA analysis she is performing on a real dataset.
• @katsanto to review the new snakemake rules to make sure that they are according to the snakemake standards.
• @bakma Check is there are further changes required, in order to integrate the output of the PCA analysis in multiqc.
• @kanitz have a look at zpca and work on the merge request.

Thank you in advance.