Added the cbrt function (the principial cube root)
Fix tests for cbrt
Fix cbrt printing
Fix docstring
Mention real_root in docstrings
Alter sqrt/cbrt docstring examples to test an equality
Revert cbrt printing
Revert "Fix tests for cbrt"

This reverts commit c9af0d8d3c2be305e9d028418de69dfb8bcaacf2.
Merge branch 'master' into 4071-cbrt
Fix printing in doctests
Add tests
Merge pull request #2577 from skirpichev/4071-cbrt

Issue 4071: add cbrt function