This will eventually allow to not hard-wired the Dirichlet vertices
into the C++ code, and change them without recompiling.  This also
means we can remove some preprocessor switches.

Currently, though, the Python code is still hard-wired, so we are
only half-way there.

Note that this patch may break several test problems.  I did not test
all of them.

[[Imported from SVN: r9793]]