- Added solveMultiCell() virtual method.
- TransportModelInterface::reorderAndTransport() now calls solveMultiCell()
instead of aborting if encountering multi-cell components.
- Implemented solveMultiCell() in TransportModelTwophase by solving
each cell individually with solveSingleCell() and repeating until
saturation change is small (hardcoded 1e-9 for now).