Added the cbrt function (the principial cube root)
Fix tests for cbrt
Fix cbrt printing
Fix docstring
Matrix pinv: correct XFAILed test.

Addendum to pull request 2571.  One test was marked as XFAIL, so I didn't
notice that it had been set up incorrectly.  Fixed and expanded; whenever
rank-deficient matrices are supported, this test will be ready to go.
Matrix pinv: replace indexing with .pop().

I mistakenly assumed expr.atoms returned a list, when it actually returns a
set.  Therefore, I tried to use indexing to access the one expected item, but
must instead use set.pop().
Mention real_root in docstrings
Alter sqrt/cbrt docstring examples to test an equality
Sort the args of Xor.

Complement to issue 4081.
Remove sorting from printer (Equivalent/Xor args are sorted already).
Revert cbrt printing
Revert "Fix tests for cbrt"

This reverts commit c9af0d8d3c2be305e9d028418de69dfb8bcaacf2.
Merge branch 'master' into 4071-cbrt
Fix printing in doctests
Add tests
problem with printer
removed wrong file
added test
Merge pull request #2577 from skirpichev/4071-cbrt

Issue 4071: add cbrt function
Merge pull request #2588 from jrioux/printing

Sort the args of Xor.