dict subclassing in templates and terminologies

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_