Removed _minimize(), _maximize() helper functions. 

Changed to:

@XFAIL
def test_U13():
    assert minimize(x**4 - x + 1, x)== -3*2**Rational(1,3)/8 + 1 # minimize() not implemented

@XFAIL
def test_U14():
    f = 1/(x**2 + y**2 + 1)
    assert [minimize(f), maximize(f)] == [0,1] # minimize(), maximize not implemented