Pyapp-flow

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Additions
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- Add SetGlobalVar to set variables current and parent scopes.

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Additions
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- Add default_vars option to Workflows to apply default values if one is not
already provided.

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Additions
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- Add required_vars option to Workflows to ensure that required variables
are set before the workflow is executed.
- Integrate required_vars into CLI.
- Add sensitive filter when outputting variables in flow-trace

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Additions
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- Add ``and_finally`` option to the Group (and derived) nodes. This provides a way
to execute nodes to clean up after a workflow has completed successfully or
an exception was raised.

Changes
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- testing.call_node now accepts a workflow context. This allows for inspection
of the context if an exception is raised.
- Fix the inheritance of exceptions. All RunTime exceptions are now based off
of ``WorkflowRuntimeError``.
- ``TryExcept`` now resolves nodes called after an exception taking subclasses
into account. This matches the behaviour of Python itself.

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Additions
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- Add TryExcept node to handle errors in a workflow.

Changes
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- ForEach node now shows each loop and allows for custom labels
for each loop.
- Improve indentation of workflow output.

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Additions
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- Add FeatureEnabled node to enable/disable a feature based on a
PyApp feature-flag.