Allow to propargate update of subworkflows to the workflow using this subworkflow

This may be more of a Galaxy issue, or it might be fixable within IWC — I’m not entirely sure.

The microGalaxy community has recently discussed whether to split some of our workflows into subworkflows. Splitting makes it easier to build new workflows from these subworkflows. However, as far as I know, it is currently **not possible to propagate updates from a subworkflow to the main workflows that include it**.

In practice, the main workflow acts more like a container holding all the subworkflows but is **disconnected from the original subworkflows**.

**Example:**

* We have a workflow in IWC for host contamination and another for MAG analysis.
* We then build a main workflow that includes these workflows and add it to IWC.
* If we later change a tool in the host contamination workflow, the subworkflow itself is updated correctly.
* However, the main workflow does **not** automatically reflect this change and must be updated manually, since it is disconnected from the original subworkflow.

The more workflows use a shared subworkflow the more difficult it gets to update all of them.

Start by determining whether subworkflow update propagation belongs in Galaxy or IWC, then inspect how the repository represents and includes subworkflows. Done means a shared subworkflow change is reflected in every main workflow that uses it without manual updates.

Written by the indexing model from the issue text.