added ``minimal_polynomial_sq`` and ``nthroot``

``minimal_polynomial_sq`` returns the minimal polynomial for the
``nth-root`` of a sum of surds.

``nthroot`` computes a real ``nth-root`` of a sum of surds.
modified ``minimal_polynomial`` to call ``minimal_polynomial_sq``;
solved issue 2895
code cleanup
faster minimal_polynomial for sum of surds
minimal_polynomial_sq renamed _minimal_polynomial_sq
Changed the numerical test in ``minimal_polynomial_sq``
added ``_choose_factor`` and an example of minimal_polynomial using
high precision
faster numerical evaluation in ``choose_factor``
Merge pull request #1993 from pernici/nthroot

nthroot of a sum of surds