'after' workflow dependency expression

[mlin]

openwdl/wdl#162

[mlin]

If a named call is in a scatter/conditional section, dependency should be on the appropriate Gather node. Need to make sure workflow type env links to appropriate Gathers even for calls with no outputs (env EmptyNamespace). Then during call typechecking, stash a list of gathers to later return under workflow_node_dependencies.