Further improvements have been made to parameter support for the `subpipeline` step:
- A sub-pipeline parameter now automatically becomes a model parameter if there is a project parameter defined with the same name. For example, if `[parameter foo]` is defined, then using `$foo` in a sub-pipeline automatically turns it into a model parameter. 
This can avoid repetition when using the same parameters across several different sub-pipelines. (GL#1700)
- Conversely, when a fixed value is specified *inline* in a `subpipeline` step, it will no longer become a model parameter. For example, `subpipeline('example', {foo: 'fixed value'})` would *not* create a `foo` model parameter. (GL#1731)
