Poly Reader: Renamed to tetgen_poly_reader and updated Doxygen documentation.
Accessors: Renamed dynamic_{field,accessor} to dynamic_{field,accessor}_wrapper.

The reason is that the respective classes do not provide actual accesses, but instead only
wrap user accessors using a common virtual base.
Doxygen: Lots of additional comments in the main viennagrid namespace.
Erase elements: Moved internal stuff into detail namespace, added missing documentation for public functions.