Samplesheet meta-schema

Define the meta-schema for samplesheets.

As far as I know, a samplesheet can be an array of either strings (e.g. fetchngs) or objects (e.g. rnaseq).

If we consider only CSV, then the object values must be primitives (strings, numbers, booleans). If we consider also JSON, then the object values can be anything, including nested arrays and objects.