Initial modification of directory structure and addition of pypolyhedron class

* Moved polyhedron class and associated functions to new libpolyhcsg directory
* Created CMakeLists.txt for polyhcsg subproject
* Removed python specific members from the polyhedron class
* Created pypolyhedron class derived from polyhedron class with python specific methods
* Modified top level CMakeLists.txt to reflect changes
* Modified python_wrapper.cpp to use pypolyhedron
Moved all polyhedron code into polyhcsg namespace and fixed various build issues