[Documentation] Bump version to 3.0-dev

Also require newer versions of core modules and TypeTree.
[C++11] Switch from dune_static_assert() to static_assert()

The backwards-compatibility header dune/common/static_assert.hh and the
macro dune_static_assert() were deprecated after the 2.3 release of the
core modules, so get rid of it.
[C++11] Remove fallback code for unordered containers
[C++11] Remove fallback code for compilers without rvalue references
[Whitespace] Clean up whitespace in common/function.hh
[C++11] Port to C++11-only version of TypeTree

TypeTree now requires GCC 4.5, i.e. variadic templates, rendering all
the weird macro hackery for backwards compatibility with pre-C++11
compilers obsolete. As those macros have already been removed from
TypeTree, along with the old CompositeNode that has been replaced by
what used to be the VariadicCompositeNode, we have to adjust all
TypeTree-based code in PDELab to make everything compile again.
[C++11] Fix a final occurence of code that relied on the feature macro for variadic templates
[Tests] Only try to build Eigen test if Eigen was found
[Tests] Make autotools aware that test-dg-amg requires GMP

In order for test-dg-amg to compile, we need to set the correct CXXFLAGS
etc. We also shouldn't try to build the test without GMP.
Merge branch 'releases/2.0'

Some bug fixes to make test suite compile

* releases/2.0:
  [Tests] Make autotools aware that test-dg-amg requires GMP
  [Tests] Only try to build Eigen test if Eigen was found