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/setup.py

@@ -0,0 +1,26 @@
+# this only installs the LAMMPS python package
+# it assumes the LAMMPS shared library is already installed
+from distutils.core import setup
+import os
+
+LAMMPS_PYTHON_DIR = os.path.dirname(os.path.realpath(__file__))
+LAMMPS_DIR = os.path.dirname(LAMMPS_PYTHON_DIR)
+LAMMPS_SOURCE_DIR = os.path.join(LAMMPS_DIR, 'src')
+
+def get_lammps_version():
+    with open(os.path.join(LAMMPS_SOURCE_DIR, 'version.h'), 'r') as f:
+        line = f.readline()
+        start_pos = line.find('"')+1
+        end_pos = line.find('"', start_pos)
+        return "".join(line[start_pos:end_pos].split())
+
+setup(
+    name = "lammps",
+    version = get_lammps_version(),
+    author = "Steve Plimpton",
+    author_email = "sjplimp@sandia.gov",
+    url = "https://lammps.sandia.gov",
+    description = "LAMMPS Molecular Dynamics Python package",
+    license = "GPL",
+    packages=["lammps"]
+)