QC rework

I restructured the quality control process and all the underlying functions.
This should fix #10 (closed).

TODO:

• utilities.load_anndata
• utilities.saving_anndata
• marker_genes.label_genes
• analyser.qcmetric_calculator
• qc_filter.set_def_cuts -> qc_filter.find_thresholds
• qc_filter.refining_cuts -> refine_thresholds
• qc_filter.anndata_filter
• plotting.qcf_ploting -> plotting.qc_violins
• adjust notebook 2
• write tests
• add logging aka add to adata.uns["infoprocess"]

Notebook overview: