TPX_Error is now derived from CanteraError, and errors in the tpx calculations
propagate automatically without requiring explicit checks for an error
condition.
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.
