Add sample sets

[jerowe]

Sometimes a sample does not exactly the specified structure, or we have multiple indirs.

Introduce sample sets, and/or multiple indirs.

- sample_set:
   - set:
       - indir: 'dataset/1'
        - sample_rule: '(Sample_.*).fastq
    - set: 
       - indir: 'dataset/2'
       - sample_rule: '(Sample_.*).fastq