FGMRES

Contents:

• added FGMRES solver to hyteg/solvers
• bugfixes related to boundary conditions for the GMRES solver
• added a callback functionality to both solvers
• added convergence tests for P1,P2 and simple Stokes cases

Note: No asymmetric test yet. For a simple test with an asymmetric test the \frac{\nabla \rho}{\rho} \cdot u operator is still missing in master.