New module savana/cna

[manascripts]

This PR adds a new module savana/cna.
Because it requires an adequately large test dataset for copy number fitting @cortés‑ciriano‑lab/savana#110, only a stub test is included.

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[SPPearce]

I really don't like the stub only test here.
Is there anything in the sarscov2 data that could be used? https://github.com/nf-core/test-datasets/tree/modules/data/genomics/sarscov2/nanopore/bam

[manascripts]

@SPPearce, as @helrick mentioned here - the 2 MB bam would be too small for the copy number fitting. Plus, there are issues running the tool using a single contig fasta. I have used this tool as a local module in the past and it worked alright with a full-fledged dataset. Do you have any suggestions for this case?