Instationary stuff not fixed so far.

The previous name was *really* confusing.

Much more telling name...

Now working apart from iteration order deterministicness

Useful, if you are only interested in the jacobian_apply versions
anyway.

It's completely unmaintained andI doubt that it will
at some point what it was intended for...

There is way more reasons to interpolate a function into the
solution vector and we cover them with one code path:
* Dirichlet boundary conditions (strong ones)
* Initial conditions for instationary expressions
* Initial guesses for Newton solvers

The visitor state regarding indices needs to be correctly reset
for child iteration in these cases.

These might sound completely silly, but they are useful when writing
code generic w.r.t. the number of components

It has single precision support...