Main code rewritten with truncation of the "empty" cells with vacuum
to handle (approximately) Riemann problems with Suliciu solver
in a "systematic" matricial way.
Also:
- testcase 2 (Ritter) in addition to testcase 1 (Stoker),
  with passive tracer
- main loop splits sums of fluxes & their addition through order-1 time scheme
  with a view to treating 2D problems by ADE (necessary for adaptive dt)