MultiQC custom content for nf-core/deepmutscan's specific QCs #100

As suggested by @maxulysse in #54: per-library QC metrics are currently in deepmutscan_report.html. In the future, these may also enter MultiQC as custom contents, so that they appear in multiqc_report.html and in multiqc_data/ for aggregation across runs. Should improve maintenance by @ewels & co.

• General statistics per DMS library: library variants observed/expected, coverage of the programmed library, reads merged and aligned, whether a library was error-corrected or not
• Error-correction section: estimated error rates per library; split by variants and variant type chemistry
• 96-type mutational signatures (SNV error rate improvements & mutational signature support #72), once we have built them in
• Written as *_mqc.tsv / *_mqc.yaml by the processes that already compute the values; no change to MultiQC itself
• docs/output.md: what goes into which report

deepmutscan_report.html will stay for the DMS-specific content views (fitness heatmaps, structure viewer, interactive variant inspection).