Transform LAMMPS Python module into package

- Moves lammps.py into its own package
- Imports entire module in __init__.py
- Changes both how legacy and CMake build systems install
- Added traditional setup.py for Python-only installation

Note: the CMake install target runs setup.py build and install
in a way that produces files in CMAKE_BINARY_DIR/python instead
of python/build. This is to maintain out-of-source compilation
support.

python/lammps/__init__.py

@@ -0,0 +1 @@
+from .lammps import *