cubic solver

[Albert Chan]

Root simplify returned wrong (not principle) root bug existed for XCas 1.5.0

But, it is worse on XCas 1.9.0-31 (mingw32 win32)
Result does not match phase *and* magnitude.

XCas> b := (1/256*(-√15*48*i-176))^(1/3)
XCas> polar(float(b))

1.0, -0.77627903074

XCas> simplify(polar(b))                     // good

\(\displaystyle 1\;,\;\frac{-\pi +\arctan \left(\frac{3}{11} \sqrt{15}\right)}{3}\)

XCas> simplify(polar(simplify(b)))       // bad

\(256\;,\;-\pi +\arctan \left(\frac{3}{11} \sqrt{15}\right)\)