This almost halves the assembly time for the Hesse matrix.  Nice!
Nevertheless I still keep the scalar-mode code around, hidden
behind and #ifdef.  Higher-order vector-mode in ADOL-C appears
to be not quite as reliable as the rest, so I want to be able to
easily compare results with the scalar code.

Also, unlike the scalar code, the vector-mode one again computes
the full Hessian in the surrounding Euclidean space.  Only afterwards,
that full Hessian is multiplied by the tangent space basis vectors
to obtain a Hesse matrix on the tangent space.  To obtain better
efficiency one could have ADOL-C compute the Hessian in the tangent
directions directly.  However, ADOL-C sometimes segfaults... when
doing that.

[[Imported from SVN: r9815]]

Newer versions of ADOL-C do not define 'overwrite' anymore, hence there
is no risk of macro clashing.

[[Imported from SVN: r9798]]

Instead, only let ADOL-C evaluate the Hessian in the directions
of the vectors of the orthonormal frames.  The result is the same,
but we get away with fewer calls to ADOL-C, which, in my Cosserat
shell example, shaves off about 10% of the assembly time.

[[Imported from SVN: r9779]]

[[Imported from SVN: r9572]]

[[Imported from SVN: r9571]]

Because the gradient is needed anyway when computing the Hesse matrix.
Not computing it separately a second time shaves off about 8% wall time
of each iteration.

[[Imported from SVN: r9555]]

The Weingarten map is block-diagonal for a product space.

[[Imported from SVN: r9517]]

[[Imported from SVN: r9422]]

[[Imported from SVN: r9415]]

Because it may contain a projection onto the manifold, which
is part of the energy evaluation if we view the energy as
being defined on all of R^n (as ADOL-C does).

[[Imported from SVN: r9414]]

Not actually working yet, though.

[[Imported from SVN: r9412]]

[[Imported from SVN: r9395]]