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'.
Fix variable declarations

* Reserve elements in pore_vol vector
* Make variable sat_diff_cell a const
Merge pull request #64 from laods/avgdiff

Optional steady state criteria