[problems.elliptic] update vector notation
* i.e. vectors are now [a b] instead of [a; b] to support matrices
[dirichletconstraints] reenabled clang guard
* this blew for me with clang 3.4
[fem.algorithm]
put everything in a discretization parameterized with the space and
la backends
[elliptic_fem]
atm we need a non const grid ptr, since the provider in stuff is not const
yet
[fem.algorithm] switch space to use dune-fem as backend^^