[#51] Finite discriminant paths seem to work.

A path to a discriminant place seems to work. Only
preliminary tests have been done because of Puiseux
performance issues. In particular, computing Puiseux
series of places at an x-point where the symbolic expression
contains a lot of terms.

Here's an example where it works:

    import abelfunctions as ab
    from sympy.abc import x,y
    f = -x**7 + 2*x**3*y + y**3
    X = ab.RiemannSurface(f,x,y)
    b = X.discriminant_points(exact=True)
    a = b[3]              # the disc point at x=a
    places = X(a)         # there are two places at x=a
    gamma = X.path(P[1])  # P[1] is 2-ramified

However, a different choice of x=a causes the path
construction to hang.