[dune-pdelab] solver for nonoverlapping grid decompositions

[Christian Engwer]

Hi Bernd,

> last time I visited you in Heidelberg, Peter gave me the idea to improve
> the available solver for the nonoverlapping case. I meanwhile
> implemented it, the result is attached in form of a patch for
> istlsolverbackend.hh, and a patch for nonlineardiffusion.cc from the
> howto for testing it.

first of all, thank you for your work. From a first glance it looks
good. Still there are some small things that should be sorted out
before applying.

- there are some missing parts in the documentation. Many methods are
  documented, but often the description of parameters is missing. Also a
  documentation of the classes as a whole is missing (especially
  ISTLBackend_NOVLP_BCGS_SSORk). I know that there are other classes in
  the file with the same problem, but we are trying to improve the
  documentation and I would rather not add even more construction
  sites. 

- Joe fixed the indentation of the file. It would be nice if you could
  update your editor accordingly. (although this would be easily fixed
  before by myself, thus it isn't very important)

Everything else looks good.

Cheers
Christian