Configure pavement to use cythonize for build

Notes
=====

 - Cython `pyx` processing is only done if the `CYTHON_BUILD`
   environment variable is set.  Otherwise, build is attempted using
   existing `.cpp` files.  This allows installation on systems without
   Cython _(or with a different version of Cython)_ installed.
Add missing file: transform.pxd
Add DescribeSummary Cython ext type, notebook