added workflow_import to StepSerializer to support single-step workflows
added workflow_import to StepSerializer to support single-step workflows

StanfordBioinformatics/loom,StanfordBioinformatics/loom,StanfordBioinformatics/xppf,StanfordBioinformatics/xppf,StanfordBioinformatics/xppf,StanfordBioinformatics/loom,StanfordBioinformatics/xppf,StanfordBioinformatics/loom