Add a Github Actions Workflow for integration testing

[dkfellows]

Working towards #346

Currently this just runs the hello and groups workflows from LightForm-group/matflow-user-documentation.

[gcapes]

This is a good step in the right direction. I'm not sure how to test this but I can see a couple of issues/improvements:

1. This is pulling workflows from another repo. I think they should live within the MatFlow repo. Indeed the groups one already does, and could be extracted from inline code to a separate code file which I guess could then be run by this action?
2. The hello.yaml workflow uses a python script and needs a full path to it. https://github.com/LightForm-group/matflow-user-documentation/blob/master/hello.yaml#L22. I'm not sure how you get round that but again perhaps if it was a part of the matflow repo, then after cloning in the action, we could specify a path? I can't think of a nice implementation but I guess you could do something with sed to replace the file path?

[dkfellows]

I've raised that path problem as a bug. I'll have a think about how to hack it... though I don't like that!

[dkfellows]

Surgery, thy name is PERL!

      run: |
        perl -pi -e 's/$ENV{REPLACE}/$ENV{PWD}/g' *.yaml
      working-directory: userdocs
      env:
        REPLACE: /home/mbexegc2/projects/lightform/matflow-user-documentation

[dkfellows]

And with that change, we now have the integration test of (some of) the user documentation workflows working.

[dkfellows]

@gcapes My intention would be to add more bits to this to test other key workflows. The one that exists explicitly is to test the user documentation repo.