Added dV_comoving function and tests.

The dV_comoving function allows for an easy method of integrating
over a comoving volume. It is useful when, for example,
integrating a function that depends on another function whose
sensitivity changes with redshift.
Renamed function to differential_comoving_volume

Changed relevant doc_strings and tests as well. As discussed at
https://github.com/astropy/astropy/pull/2103.
Changed documentation wording.

Changed docstring and added the change to the CHANGES.rst file.
Also added the function to the __all__ list at the top of funcs.
Changed the return with units method.

In response to comment here:
https://github.com/astropy/astropy/pull/2103#commitcomment-5470863
Changed to long form of steradian.
Updated _get_current function call.

The _get_current() is depreciated so changed to the new:
 _default_cosmology.get()
Merge pull request #2103 from jbwhit/feature-comoving-volumeelement-addition

Added dV_comoving function and tests.