cornerAngle() doesn't work for polygonal faces #214
Description
Attempting to invoke geometry->cornerAngle(c) on a non-triangular face yields the message
GC_SAFETY_ASSERT FAILURE from /Users/keenancrane/Code/mesh-mash/deps/geometry-central/src/../include/geometrycentral/surface/vertex_position_geometry.ipp:67 - faces must be triangular
This seems like an oversight, since the angle at a corner is well-defined for a face of any degree (even if the geometry within the face is not well-defined in the nonplanar case).