Minor bugfixes to the PlanarGraph class

Three small fixes for bugs found in PlanarGraph are made (split off from !79):

1. Ensure that the correct sort order is maintained in the presence of departure azimuths of \pm\pi at a vertex.
2. Fix a coding error whereby the stitching flag was not set for a potentially non-contiguous path append operation.
3. Fix an unwanted copy where a reference was meant to be taken.