LF's only breaking change over the past two versions has been the
deprecation of `lang=None` as a valid parameter. This is because the new
language loading paradigm wants to load certain functions on the fly,
which it cannot do when it is explicitly told to look for a null lang.

I've addressed this by passing `lingua_franca.get_default_lang()` where
the `lang=None` call remained.

Bonus: Gets rid of over 200 DeprecationWarnings in unit tests!