Improvement/task and spec relations

[essweine]

This PR

• adds attributes for storing inputs and outputs as spec names on task specs with properties for returning the objects
• adds a tasks attribute to the workflow that allows access to tasks by id rather than traversing the tree
• a little miscellaneous cleanup around serialization

The purpose of these changes is to make deserialization simpler (it's no longer necessary to reconnect all the task specs during deserialization, and tasks can be deserialized independently of constructing the task tree).