Fix #2559: Make equivalencies work correctly with compose.
Add another compose-to-equivalencies example.
Fix tests
Default ``data_start`` to 0 in ``CommentedHeader``

fixes #2692
Ensure Quantity.round always works; wrap methods with less boilerplate
Adapt places where "wrong" np.round led to the right result
Add changelog entry [skip ci]
Ensure test_std_inplace always fails, as it is broken in numpy
Avoid keyword arguments where possible to resolve numpy 1.5 failures
Update _wrap_function doc string, correct and test nansum out option
Early experiment in giving names to model input/output coordinates, rather than just counts of inputs/outputs.
Break _ModelMeta.__new__ into a few subroutines for readability.  Brings the same code generation used by custom_model to the main Model class itself for __call__ methods.  Now it's generally unnecessary for Model subclasses to redefine __call__ even if they want to update the method signature to reflect the names of their inputs.  That is now automatic.  Also the Model base class no longer makes any assumptions about the number of a model's inputs or outputs.  It assumes no inputs/outputs until otherwise specified in a subclass.
Fix some minor issues with the array-like interface to Parameters that came up, interestingly, when trying to call help() on a model class.
Remove the custom __call__ overrides in these projection classes which are no longer needed due to the automatic code generation in the _ModelMeta metaclass.
Extend the code generation performed in _ModelMeta.__new__ to include generating __init__ methods as well.  This allows still more boilerplate to be removed from most Model subclasses (though it still allows individual subclasses to define their own __init__ when needed--see for example the polynomial models).
Remove the __init__ and __call__ generation from the custom_model decorator, since that part is now already handled (and somewhat better) by _ModelMeta.__new__
Fix typo in decorator per https://github.com/astropy/astropy/pull/2835/files#r16432656
For some reason the doc build is complaining about not having pages for the Pix2Sky/Sky2Pix base classes.  Unfortunately the only obvious way to add them is to include them in the module's __all__ even though they're not generally useful for users (though I guess useful as base classes for new projection types?)  If there's another way to do this let me know.  This is okay in the meantime though.
Made Latex/AASTex tables use unit attribute of Column for output
ModelMeta now creates a nicer __init__ definition for models that have no parameters.