use pythonic options instead of _ad hoc_ name variations for igl function overloads

[alecjacobson]

There a bunch of these and maybe later I'll try to enumerate them all.

For example in C++ igl::boundary provides these two overloads:

 template <typename DerivedF, typename Index>
  IGL_INLINE void boundary_loop(
    const Eigen::MatrixBase<DerivedF>& F, 
    std::vector<std::vector<Index> >& L);
  template <typename DerivedF, typename Index>
  IGL_INLINE void boundary_loop(
    const Eigen::MatrixBase<DerivedF>& F, 
    std::vector<Index>& L);

The first returning all boundary loops and the second returning the longest. In the current python bindings these manifest as igl.all_boundary_loop and igl.boundary_loop.

To avoid confusion (see, e.g., #67, #124), the python binding names should match as closely as possible to the underlying C++ function.

The pythonic solution would be to use kwarg optional arguments like

igl.boundary_loop(F) # defaults to return_all=False
igl.boundary_loop(F,return_all=True)

I'm not sure yet how to set these up with pybind11 but it seems possible.