Test cases 1x:

- no phase transitions (always sub-saturated solution)


Test case 1-a:   diffusion_cell_1a.d6p

- diffusion cell
- initial condition = bc
  -> initial solution content and boundary solution content -> no flux
  -> all solution properties correctly computed



Test case 1-b:   diffusion_cell_1b.d6p
- diffusion cell
- concentration gradient (variable) at boundary, diffusion flux within
  cell
  options: 
    - no density effect <-> density effect causes convection
    - counter diffusion of water <-> no counter diffusion



