add multiprocess + lots of fixes and improvements

The main improvement is multiprocessing for match, delta, spdxdebian and spdxalien steps (the other steps cannot usefully use multiprocessing: 'add' step is already super-quick, 'scancode' already uses multiprocessing when launching scancode command, and upload must function sequentially, otherwise it could overload fossology server with too much requests -- the latter would need some testing, though; maybe "controlled" multiprocessing may function).

Another important improvement is primary archive support in AlienMatcher class, so all subsequent steps can work on the primary archive of each alien package

The remainder commits implement minor fixes and improvements