This PR attempts to reorganize the Cython wrapper using pxd/pyx separation. The goals are:
easier to read & manage the source
possibly faster build time when changes are local
(most important to me) ability to cimport the base classes in order to define custom factors in user code, without having to rebuild the AD3 python wrapper.
This PR attempts to reorganize the Cython wrapper using pxd/pyx separation. The goals are: