Implement `InheritDocstrings` metaclass to automatically inherit
docstrings on methods from those in the base class.
Add warning about multiple inheritance.
Move io.votable to use InheritDocstrings
Move units to use InheritDocstrings
Inherit ndarray docstrings in Quantity
Merge pull request #2159 from mdboom/misc/docstring-inheritance

Automatically inherit docstrings from base classes