Replace AztecOO by Belos

Trilinos has several packages for the iterative solution of linear equation systems. The historical one, AztecOO, currently interfaced in FEniCS through EpetraKrylovSolvers.*, falls behind the more modern package, Belos, in several areas. One particular aspect is the absence of a MINRES implementation in AztecOO.

Replace AztecOO call by Belos in EpetraKrylovSolvers.*.

Once MINRES is available, use it in demo_stokes-iterative instead of QMR.