Signed heat method

[nzfeng]

Implements the Signed Heat Method for computing signed distance described in the paper "A Heat Method for Generalized Signed Distance".

Summary of the contributions:

• Adds the class SignedHeatSolver that acts on triangle meshes. The class is stateful like that of VectorHeatSolver, etc.
• Adds the function computeSignedDistance() to the class PointCloudHeatSolver that implements the SHM on point clouds.
• Adds documentation for both the triangle mesh and point cloud solvers.
• Tweaks computation of the edge rotations for crouzeixRaviartConnectionLaplacian in intrinsic_geometry_interface.cpp, to use an edge lengths-based formula, instead of one based on corner angles.
• Fix a small bug in the SurfacePoint object.