[FIX] More reliable group-reports stage
[FIX] More reliable group-reports stage

This PR wants to fix two problems:

  1. The group level failed when no anatomical qc-json files were
     found in the derivatives folder. Now, it will only issue a warning

  2. The group level will automatically be executed if no --participant_label
     was specified.

Closes #196

poldracklab/mriqc,oesteban/mriqc,oesteban/mriqc,poldracklab/mriqc,oesteban/mriqc,poldracklab/mriqc,oesteban/mriqc,poldracklab/mriqc