BIDSLayoutDeprecationMixin?

[adelavega]

Would it make sense to have a BIDSLayoutDeprecationMixin which handles all of the deprecation warnings in __init__ and beyond?

This should wait until we're done w/ the refactor, but would make the code cleaner.

[erdalkaraca]

I think the mixin mechanism (monkey-patching in Python) can only replace/add properties of an object, but it will be hard to modify the behavior of an existing class function/method/constructor.

[adelavega]

We could do a decorator