Add the binding for getting all of the boundary loops

[QhelDIV]

Hi, thanks for the great work! (I am using libigl in a daily basis)

In the cpp implementation the boundary_loop function has two overloads: One returns all of the loops, the other only returns the largest loop.

It seems in the python-binding only the latter overload has a python API. I wonder if it is possible to also expose the first overload to python? Currently, I have to use igl.boundary_facets first and use scipy's sparse graph utilities to do the same thing. If the libigl python binding can support this, it would be much more convenient, thank you!