make .get() and indexing ([]) more pythonic

[keighrim]

Because

At the moment, various classes in mmif.serialize package implements .get() and .__getitem__() methods, but their behavior are

1. not consistent across classes
2. not well-documented in one place (they all have decent docstring, but no aggregate of them exists)
3. not so "pythonic" in that their behavior often deviates from standard python get and __getitem__ methods in built-in classes.

Done when

• all existing (de-)serialization classes are categorized into "dict-likes" and "list-likes"
• inspect current implementation of get and __getitem__ for each
• update, if necessary, the methods to match those of python dict's and list's

Additional context

This proposed change will vastly break compatibility of the SDK to existing CLAMS app code, so we need a version marker that can mark this breakage. (#293 )