Factor out common logic in API functions

[asmeurer]

A lot of API functions have common logic at the top of each implementation. For example, calling ndindex() on the input arguments, or type checking that is the same in every case.

We should refactor the common logic to the superclass, and have the methods call implementations in the subclasses. Something like

class NDIndex:
    def method(self, arg):
       # common logic goes here

       return self._method(arg)

    def _method(self, arg):
        raise NotImplementedError

class Subclass(NDIndex):
    def _method(self, arg):
        # Subclass specific implementation

The only downside of this is that it makes it impossible to have subclass specific docstrings. Presently reduce and expand are the only methods with subclass specific docstrings, and I don't think they have any common logic, so they can be left alone. For other methods, we could have some way to automatically replace the docstrings with docstrings on the implementation methods.