Adds a Python test script.

This script tests all 3x3 geometric configurations to make sure that
SSAFD does not crash because of "ice noses" and such.
Adds a Maxima script to support recent changes.

This script (together with test_ssafd.py) shows that PISM's SSAFD code
can handle "ice noses" just fine and we don't need to remove them. There
are only 16 configurations that cause SSAFD failures:

X O X
O I O
X O X

i.e. an icy cell (I) with zero basal drag (floating, for example) with
four ocean neighbors (O). It does not matter whether diagonal
neighbors (X) are icy or ice-free.

In these 16 cases the implementation of the calving front boundary
conditions (CFBC) assembles the SSAFD matrix with zeros on the diagonal
for all 4 equations (2 equations for the u component and 2 for v).