Support submission of metagenomic assemblies generated from co-assemblies

Difficulty: Medium

Why this matters: the pipeline submits metagenomic assemblies to ENA (European Nucleotide Archive). A co-assembly is when reads from several sequencing runs are assembled together into one set of contigs, instead of assembling each run on its own. This is common in metagenomics because it can improve results for organisms that are rare in any single sample. Right now the metagenomic_assemblies mode assumes one run per assembly, so there's no way to describe "this assembly came from runs A, B and C together". This is the other half of the co-assembly problem, on the metagenomic assemblies side (see issue # for the equivalent problem on the MAGs/bins side).

Good to know before starting: there's already an open pull request (#66) that implements most of this, it accepts semicolon-separated run accessions and read files, calculates coverage across multiple runs, and registers a stand-in sample that ties the co-assembly back to all its source runs. The task here is to pick that PR up and finish it rather than start from scratch.

Some terms: ENA (European Nucleotide Archive) is the public repository this pipeline submits data to, and Webin is ENA's account and authentication system needed to submit anything (free, self-service registration). Co-assembly is explained above. An "umbrella" or virtual sample is a synthetic ENA sample used to represent data that comes from more than one original sample, since ENA submissions need to point at exactly one sample.

Skills / experience: Nextflow, reading and continuing someone else's in-progress code, nf-test (see the nf-core nf-test testing guide if this is new to you). Familiarity with the concept of genome coverage depth would be helpful.

Suggested approach:

1. Read the metagenomic_assemblies mode section of docs/usage.md to be familiar with submission process.
2. Check out the PR #66 branch and read its description, which already lists what's left to do.
3. Fix the coverage calculation bug it flags (the multi-run CoverM output has more columns than the single-run case handles).
4. Help get the PR reviewed and merged.

Read the metagenomic_assemblies section of docs/usage.md, then check out PR #66 and review its description for the remaining work. Investigate the multi-run CoverM output and its coverage calculation, using nf-test as needed. Done means the co-assembly submission flow is tested and PR #66 is reviewed and merged.