Various improvements to using sub-pipeline steps, which make it easier to 
reuse and share model pipeline components. These changes include:
- The ability to define a `[subpipeline]` in an INI file. 
The `subpipeline()` pipeline step now accepts an `id` argument that can contain _either_ the ID of a `subpipeline`
configuration, or a filepath. These changes now make it easier to share and import sub-pipelines across different projects.  (GL#1657)
- The INI file sub-pipeline configuration can also include default values for the sub-pipeline's parameters.
This effectively makes supplying sub-pipeline parameters _optional_ in some cases. (GL#1670)
- Model parameters are now automatically passed through to the `subpipeline` step.
This avoids needing to explicitly "plumb" through parameters in the pipeline code - for example,
you can now specify `subpipeline('foo')` instead of `subpipeline('foo', { bar: $bar })`. (GL#1656)
- Refer to :ref:`subpipelines` for more details on using the latest sub-pipeline functionality.
