Updates to bounding box interface and cleanup - #4264

a bounding box on a model.  The format of the bounding box itself
(as a tuple of tuples) is not changed.  This is motivated in large
part by wanting the API to more closely resemble the API for inverses
which is somewhat simiar--model.inverse is a property of a model that
returns something based on a model instance *if* the inverse is defined
on that model's class.  Otherwise it raises NotImplementedError.  Regardless
of whether a default inverse is defined on the class, the user can assign
their own inverse to any model instance.  The user-defined inverse can
unset (as of PR astropy#4236) by calling del model.inverse, which will reset it
back to the inverse defined at the class level.

This PR makes bounding_box work entirely analogously to the above.
The bounding_box_default() method is removed--instead, in order to
define a bounding box for a specific model class, simply define a
property called bounding_box on the class (the metaclass will then handle)
wrapping that property in the standard getter/setter for Model.bounding_box
defined in the Model base class).

The one extra complexity for bounding_box is that we want *some* cases
of bounding_box to be *callable* with adjustable parameters (such as
how Gaussian1D.bounding_box_default() takes an optional "factor" argument).
To this end, the object returned by model.bounding_box is actually a
subclass of tuple that looks like a normal bounding box tuple.  But on
a model whose bounding_box takes arguments (and is a method instead of
a property) one can *call* model.bounding_box like
``model.bounding_box(factor=1.2)`` to adjust the bounding box.

…interface, and some minor code style cleanup to go with it.