Temporary checkpoint
[#49] Remove need for dummy Riemann Surfaces

An earlier design required that a RiemannSurface object be
used to instantiate a puiseux series. This required several
"dummy Riemann surfaces" be created in integral_basis,
singularities(), and differentials(). Upon further
inspection, the use of a RiemannSurface object became
unnecessary since only the curve f = f(x,y) is ever used
by these functions.

Resolves #49.