Refactor `sghi.etl.commons.sources.GatherSource` to delay the disposal
of embedded sources. All embedded sources will be disposed of when the
`GatherSource` is disposed of. This will support streaming sources that
may need to remain "live" even after their `draw()` method returns.

Refactor all composite processors in the library to delay the disposal of
their embedded processors. All embedded processors will now be disposed of
when their parent processor is disposed of. This will support streaming
processors that may need to remain "live" even after their `apply()`
method returns.

Refactor all composite sinks in the library to delay the disposal of
their embedded sinks. All embedded sinks will now be disposed of when
their parent sink is disposed of.

A `WorkflowBuilder` is a class that aids in the structured construction
of `WorkflowDefinition` instances. The builder class offers a convenient
way to construct SGHI ETL Workflows by providing methods to register
sources, processors and sinks.