In particular, this fixes paths for Linux distros that put Python modules in
'lib64/pythonX.Y/site-packages'.
Also fixes the reported installation path for the Python module on Windows.
Setting the scons options 'python_prefix=USER' or 'python3_prefix=USER' will
pass the '--user' flag to setup.py, causing the Python module to be installed to
a platform-specific directory on the user's module path.
These changes make it unnecessary to copy header files around during
the build process, which tends to confuse IDEs and debuggers. The
headers which comprise Cantera's external C++ interface are now in
the 'include' directory.
All of the samples and demos are now in the 'samples' subdirectory.