update setup.py

Author: mscroggs

setup.py

@@ -28,7 +28,7 @@
         author_email="[email protected]",
         maintainer_email="[email protected]",
         url="https://github.com/mscroggs/symfem",
-        packages=["symfem"],
+        packages=["symfem", "symfem.core", "symfem.elements"],
         data_files=data_files,
         include_package_data=True,
         install_requires=requirements