Allow for `kind` kwarg to Table.argsort
Use mergesort alg for groups sorting to maintain order
Fall through to default quicksort if mergesort fails
Add unicode and latex aware liter symbol
Don't install wcs header files that aren't part of the API.
Reorganize units docs to emphasize Quantity over Unit.
Elaborate on uses of CDS unit standard
Use a mixed-unit example for Quantity-instantiated-by-list
Fix undefined reference target errors
Force stable sort for grouping
Merge pull request #1599 from taldcroft/table/group-mergesort

Force stable sorting of table in group_by()
Update CHANGES.rst

Remove unnecessary changelog entry.  See https://github.com/astropy/astropy/pull/1613/files#r7163815
Merge pull request #1591 from mdboom/unit/reorganize-docs-to-emphasise-quantity

Re-order documentation for astropy.units
Merge pull request #1618 from mdboom/unit/liter-symbol

Add unicode and latex-aware liter symbol
Merge pull request #1631 from mdboom/wcs/dont-include-non-api-headers

Don't install wcs header files that aren't part of the API
Trial with equivalency=(u.rad,None)
Add radian_is_dimensionless function to equivalencies.py
Add test cases, and, as a result, correct is_equivalent
Also rename equivalency to angles_dimensionless
Add documentation for angles_dimensionless in units/equivalencies.rst
Correct equivalency tests