[WIP] Feature/solverfactory

This MR mainly contains the factory implementation from !5 (closed) with some cleanup:

• Allow creating solvers / preconditioners from ParameterTrees.

• A solver (including preconditioner) can now be configured via an ini file like this:

[solver]
precond = SeqSSOR
solver = CGSolver

[SeqSSOR]
iterations = 1
relaxation = 1.8

[CGSolver]
reduction=1e-9
maxit = 5000
verbose = 3

• The corresponding dune code currently looks like this:

auto solver = SolverPrecondFactory::create<VectorType> (matrix, configuration, "solver");

• Both the actual type of the solver/preconditioner and their parameters can chosen in the ini file.
• It is also possible to create only a solver or only a preconditioner this way. See the factory doxygen for details.

This MR depends on !84 (merged).

Note: change target branch to master, once !84 (merged) is merged!