Optional steady state criteria

Instead of using the maximum cell saturation difference as criteria
for stationarity, use a norm equivalent to the Eulidean 2-norm scaled
with pore volume. In this way stationarity is not ruined by some possibly
small, degenerated cells.

The old criteria is still the default, but the new can be used by setting
input parameter 'use_maxdiff' to 'false'.

This PR handles issue #56. Comments are welcomed!