Add tests (and fix implementation for) EBU-TT-D style conversion

[ghost]

This checks that, when converted from EBU-TT-Live to EBU-TT-D, the styling > style elements do not contain nested references. Instead the styling information contained in referenced styles should be merged with the parent style, keeping the original style ID.

So if there are 4 style elements like so:

<style id="s1" ... />
<style id="s2" style="s1" ... />
<style id="s3" ... />
<style id="s4" style="s2 s3" ... />

Then the converted document will contain style s4 with all the elements of s3, s2 and s1 with priority to the attributes of s4 > s3 > s2 > s1.

This PR also moves padding defined in <style> elements to their corresponding <region>

[nigelmegitt]

Some pretty minor changes requested, this mostly looks good.

[nigelmegitt]

I just noticed that when I run make test on this branch the new tests aren't run. I'm not sure why test_ebuttd_required_metadata.py and test_ebuttd_style_references.py are not picked up. My terminal output goes straight past them. Here's an excerpt:

testing/bdd/test_documentMetadata_elements_order.py ..........                                                                                                                                          [ 19%]
testing/bdd/test_elements_active_times.py ..............................................................................................                                                                [ 32%]

[nigelmegitt]

Sorry, my bad, they're running fine.

[nigelmegitt]

Looking really good, sorry I've added a few "down in the weeds" comments to keep things tidy, but this functionally works, so once those last minor tweaks are done this will be good to merge.

[nigelmegitt]

Thanks for addressing the previous review comments @danpope-bbc ! Looking good to squash and merge, I think.