Adjust ISTL support code to new vector creation TMP interface
[Constraints] Adopt new, ContainerIndex-based constraints implementation from PDELab

- Use local constraints container(s) instead of global one
- Commit local constraints to global ones in assembler
- Export correct constraints container from CouplingGridFunctionSpace
- Update call to constrain_residual
[DataHandle] Implement data handle support for MultiDomainGridFunctionSpace

This patch adds data handle support based on the new backend API to MDGFS. Unfortunately,
I didn't figure out how to do this as smoothly as in the standard PDELab case, where we
don't even need the GridFunctionSpace for this anymore, but can operate exclusively on
the ordering. The problem with that approach is that in the MDGFS, I need to convert between
the MultiDomainGrid and possible SubDomainGrids, which requires access to the GridView and
thus to the GridFunctionSpace.

So I unfortunately had to copy over the DataHandleProvider from vanilla PDELab and adapt
it to the multidomain scenario. Still, the implementation tries to dispatch to the vanilla
version as early as possible by only handling the direct child level of the MDGFS. Differentation
between the different types of child spaces is based on tag dispatch on the flavor tag as usual.
[CouplingLocalFunctionSpace] Fix up _dof_indices pointer in constructor before calling setup()

This patch fixes the same problem with a bogus pointer that
has been bothering PDELab as well and which only shows up with
clang for some weird reason (bad stack reuse by gcc?).
[SubProblemLocalFunctionSpace] Port synthethized multi-component SubProblemLFS to new index API

The variant of SubProblemLFS formed by picking multiple children
of MultiDomainGFS, creating a synthetic local space which does
not really exist in the tree, was not ported over to the new backend
and index API yet.

Apart from doing most of this porting (there might still be some
problems with types in the traits class), this patch also contains
some improvements:

- Add a correct maxSize() implementation
- Remove weird template parameter from localIndex()
- Avoid storing re-ordered copy of DOFIndices. This change slightly
  increases the cost of calling dofIndex() (we now need to do a two-level
  lookup), but saves some fixed time (and space) overhead during space setup.
  And as I can't think of a lot of situations in which I would want to
  call dofIndex() on a SubProblemLFS, the tradeoff seems to be better when
  more in favor of fixed setup costs.
[DOFMapper] Shut up clang warning about having to specify return type of a multi-statement lambda
Merge branch 'master' into next

* master:
  [DOFMapper] Shut up clang warning about having to specify return type of a multi-statement lambda
[EntityWrapper] Make EntityPointer typedef in SkeletonIntersectionWrapper public

The typedef for the EntityPointer in SkeletonIntersectionWrapper should be
public, just like the equivalent typedef in the IntersectionGeometry class
from plain PDELab.
Merge branch 'master' into next

* master:
  [EntityWrapper] Make EntityPointer typedef in SkeletonIntersectionWrapper public
[Buildsystem] Replace obsolete header AM_CONFIG_HEADER with AC_CONFIG_HEADERS
Merge branch 'master' into next

* master:
  [Buildsystem] Replace obsolete header AM_CONFIG_HEADER with AC_CONFIG_HEADERS
[PDELab] Adapt to numerous changes in include header organization within PDELab
[LFSIndexCache] Implement LFSContainerIndexCache -> LFSIndexCache rename from PDELab
[AssemblerEngine] Export constraints caching requirements
[GridOperator] Port to unified constraints access interface in assembler engines

The assembler engines of the vanilla PDELab GridOperators now provide direct access
to the constraints of the underlying spaces, but that interface uses shorter function
names than what MultiDomain has been using. Adapt to PDELab for consistency and
interoperability with OneStepGridOperator.
[CouplingGFSOrdering] Adapt to new base class interfaces with GlobalDOFIndex
[GridOperator] Add BorderDOFExchanger in analogy to vanilla PDELab
[GridOperator] Make sure to only call communicate() if the problem is really parallel

This is important to avoid YaspGrid barfing up for serial problems with DOFs on faces.
[SubProblemLFS] Fix compilation problem
[CouplingGridFunctionSpace] Adapt template parameters

CouplingGFS now uses the default ISTLVectorBackend by default instead of the
removed StdVectorBackend. It now also takes the OrderingTag as an additional,
but defaulted template parameter.