Embedding the Python interpreter multiple times in the same process can
cause this issue due to import caching. https://bugs.python.org/issue27400
This seems to be avoidable by using the time module instead.
- 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.