and solve() is chosen.  So far, this was done be specializing  auxiliary helper classes.
I replaced this with simple if (n==...).  One would think that a compiler should be able
to optimize this away.

Indeed: measurements using g++-4.1 showed that there is no speed difference when compiled
with -O3.  When compiled without optimization, the new code is actually about 25%
_faster_ than the old one!  Good for them users of block Gauss-Seidel!

[[Imported from SVN: r4729]]