Added tests to coordinates, constants, convolutoin, cosmology
Reapplying changes ignored during rebase
Moved xfail code to before failure-inducing code.
xfailed convolution kernel tests
Removed references to coordinates.CartesianPoints; removed unnecessary whitespace
Merge pull request #1947 from ktchrn/pickle-tests-issue-1943

Issue 1943 - systematically testing pickling of astropy classes