Namespaces

[bworrell]

This pull request addresses #224 and to some extent #200.

Updated behavior of ns_dict and schemaloc_dict parameters for Entity.to_xml():

• Namespace definitions (alias mappings) and schema locations that do not conflict with python-stix defaults are passed through to the document
• Namespace alias mappings and schema location mappings which are already found in python-stix are ignored--python-stix values are used instead.

[landscape-bot]

Repository health decreased by 0.10% when pulling 3d56f0b on namespaces into ecdca61 on master.

• 4 new problems were found (including 0 errors and 4 code smells).
• No problems were fixed.

[gtback]

I personally find the previous style more intuitive, even though it's a couple more lines. I'm ok with this, though.

[gtback]

Looks good to me. 👍 for all the additional comments, which help me actually understand what the finalize method is doing. Thanks, @bworrell