Riemann solvers for one-dimensional advection

  rp1ad.f -- constant velocity
  rp1adecol.f -- color equation with edge velocities
  rp1adecon.f -- conservation form with edge velocities
  rp1adcol.f -- color equation with cell-centered velocities
  rp1adcon.f -- conservation form with cell-centered velocities
  rp1adfw.f -- conservation form with cell-centered velocities, using f-waves