ENH: single-session workflows 

New config variable WORKFLOW_TYPE
 * atlas-based-reference (previously 'baseline') OR 
 * subject-template-generation (previously 'template') OR 
 * subject-based-reference (previously 'longitudinal')
 
Valid WORKFLOW_COMPONENTS (for *-based-reference ONLY):
 * 'tissue_classify'
 * 'auxlmk'
 * 'segmentation'