Fixed problem described in #32. There is a problem with the input channel for metabat, where assembly mappings and the assembly from wrong samples are joined.

The input channel is created by joining the following two channels:

• assembly_mapping_for_metabat.groupTuple(by:[0,1]):
  [MEGAHIT, SAMPLE_ID1, [MEGAHIT-SAMPLE_ID1-SAMPLE_ID1.bam, MEGAHIT-SAMPLE_ID1-SAMPL_ID2.bam], [MEGAHIT-SAMPLE_ID1-SAMPLE_ID1.bam.bai, MEGAHIT-SAMPLE_ID1-SAMPLE_ID2.bam.bai, MEGAHIT-SAMPLE_ID1-SAMPLE_ID1.bam.bai]]
  ...
• assembly_all_to_metabat_copy:
  [MEGAHIT, SAMPLE_ID1, SAMPLE_ID1.contigs.fa]
  ...

Those two channels are currently joined with default parameters, thus by index 0 which is the assembler name and not the sample name. This did not cause a problem for single-sample analyses or when, by chance, the order of the emitted assemblies and assembly mappings was not changed during the run. However, this resulted in cases where assembly mappings and assembly from different samples are joined and caused the in PR #53 observed error, due to wrong contigs.

Fixed this by using joinwith parameter by:[0,1] to join by assembler name and sample name.

PR checklist

• This comment contains a description of changes (with reason)
• If you've fixed a bug or added code that should be tested, add tests!
• If necessary, also make a PR on the nf-core/mag branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
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• CHANGELOG.md is updated
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