Add ExpressionKernel, base remaining kernels on that

sumpy/e2e.py

sumpy/e2p.py

sumpy/expansion/__init__.py

sumpy/expansion/multipole.py

sumpy/kernel.py

sumpy/p2e.py

sumpy/p2p.py

sumpy/qbx.py

sumpy/symbolic.py

sumpy/version.py