Allow unspecified columns in samplesheet

[ScottNortonPhD]

Currently, samplesheetToList warns about any columns in the samplesheet not specified in the schema, yet some pipelines may allow or even encourage additional metadata with arbitrary keys i.e. CRISPR gRNA sequence or organoid differentiation stage.

Proposal is to create a samplesheet schema key to control the behavior of unspecified columns, or an option to the samplesheetToList function controlling the same.

Extra columns should be included in the meta map under a subkey ie "extra", so as to avoid collision when using the "meta" schema key.

[nvnieuwk]

Hi so #157 introduces more flexible logging for these kind of warnings. You could set validation.logging.unrecognisedHeaders to skip to completely disable these warnings.

I however don't think adding a specific field for additional metadata is not a good idea since it's already possible to add optional fields to samplesheets. These additional metadata fields should just be optional fields in the samplesheet