Workcraft plugins for plugin development tutorial

This is a collection of plugins for Workcraft that are used as examples in the plugin development tutorial.

Plugins are included as submodules and, if necessary, can be used independently. Here is the list of included plugins and links to their repos:

Directed graph -- workcraft-plugin-example-graph
Petri net -- workcraft-plugin-example-petri

Suggested usage:

If has not been done yet, setup Workcraft development directory workcraft as instructed in workcraft repo.
Go to workcraft directory and clone example plugins repo with all submodules in workcraft-plugins-example directory:
```
git clone --recursive [email protected]:workcraft/workcraft-plugins-example.git
```

Note that /workcraft-*/ is prevented from propagating to workcraft repo and to Workcraft binary distribution.