Add samplesheet schema for our SDRF input

[jpfeuffer]

Description of feature

I think this is used for the nf-tower Datasets feature.
For now, I think it is a json schema like the input params (nextflow.config) but probably it would be nice to think about a PEP specification in the longterm for SDRF in general.

Example of the JSON schema in RNAseq pipeline: https://github.com/nf-core/rnaseq/blob/master/assets/schema_input.json
PEP specification: http://pep.databio.org/en/latest/