Support reading EBU-TT-D documents

[nigelmegitt]

Note gotcha: EBUTTDDocument.create_from_xml has a nasty hack workaround that tweaks the name of the document element to ttd instead of tt. Found during testing, a bug in ebu_tt_live/documents/ebuttd.py - it doesn't check for the presence of the prefix, so only works when the default namespace is the TTML namespace and no prefix is used on the tt element, i.e. fails for tt:tt

Fix is:

        if xml_dom.documentElement.tagName == xml_dom.documentElement.prefix + ':tt':
            xml_dom.documentElement.tagName = xml_dom.documentElement.prefix +  ':ttd'

In other words, test for the prefix.