workflow runtime - first cut

[mlin]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 890

• 374 of 392 (95.41%) changed or added relevant lines in 9 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.05%) to 96.873%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/_util.py	37	40	92.5%
WDL/runtime/workflow.py	274	281	97.51%
WDL/runtime/task.py	23	31	74.19%

Files with Coverage Reduction	New Missed Lines	%
WDL/_util.py	2	93.6%

Totals
Change from base Build 880:	-0.05%
Covered Lines:	4027
Relevant Lines:	4157

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💛 - Coveralls