simplify result storage at the cost of copying
replace underlying DAG task with method

This does not pass tests that use nested DAGs
output connector copies results to parent
fix system tests

This simplifies the workflow building process, though the top level
workflow is still non-uniform.
support passed-through params with parallel by
fix post failure tests
fix roundtrip tests
Merge pull request #66 from mark-burnett/dag-as-method

Convert DAGs from Tasks to Methods (#42)