Added roots_quartic_euler giving simpler solutions of the quartic equation

For a class of quartic equations the Descartes-Euler method gives simpler
solution.
added test
faster roots_quartic_euler
changed roots_quartic_euler to have deterministic output
fixed bug in doctest
renamed ``roots_quartic_euler`` to ``_roots_quartic_euler``
modified its arguments to avoid useless computations
small changes in the documentation.
Merge pull request #1947 from pernici/quartic

simpler solutions for a class of quartic equations