dict subclassing in templates and terminologies

[mpsonntag]

Subclassing the dict built-in is generally discouraged (and collections.UserDict exists for this purpose) but I think the reasons for that don't affect us here right now. I believe it's only an issue when overriding the magic methods, which we're not. Might want to keep in mind in case we do anything more fancy with the class.

Originally posted by @achilleas-k in https://github.com/G-Node/python-odml/diffs