Adding Signed Heat Method (& a bit more!)

[nzfeng]

This PR:

• Exposes the signed heat method implementation for triangle meshes (addressing python binding for Signed Heat Method #22)
• Exposes the signed heat method implementation for point clouds
• Adds all the heat solvers for general polygon meshes, in a new class called PolygonMeshHeatSolver
• Adds support for polygon meshes; in particular,
  • a new function read_polygon_mesh() in io.py returns a nested list encoding polygon faces of arbitrary degree (since a NumPy array is no longer possible in this case)
  • validate_mesh() in core.py is amended to account for this possible data structure change.
• Adds Python binding to the marching triangles implementation in geometry-central
• Adds a new edges() function in mesh.py to return the edges of a mesh
• Updates test/sample.py to also test/demo these new functionalities
• Updates the documentation in the README

[nmwsharp]

Minor nit: can we say "3D triangle meshes & polygonal meshes" (I don't want to give the impression that all the methods here specifically target polygons since some of them still don't)

[seekever]

So cool!!!