Allow input samplesheets to have additional metadata columns

[jfy133]

For many projects having a 'common' sheet that is reused across all analyses and contains all relevant metadata for all libraries/samples is useful to have.

It would be nice if we our input TSV samplesheet check can accept when the input TSV has additional columns (even if we don't use them)

[apeltzer]

Should be fair to ignore additional columns and just look for the ones we really rely on / need 💯

[jfy133]

@TCLamnidis do you think it would be possible based on what we've done so far?

[maxulysse]

We're doing that in sarek

[scarlhoff]

This should be possible with samplesheet validation by nf-validation as implemented in #1056, but needs channel manipulation magic to work.