Unify template and terminology handling

Currently there are two files, `odml/templates.py` and `odml/terminology.py`, that provide loading and handling of odml-templates and odml-terminologies.

Since the code itself is very similar, it would be good to unify the content of both files.

Refactoring should not break the current usage of odml-terminologies in [odml-ui](https://github.com/G-Node/odml-ui)