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Refactored PETSc sparse matrix and vector classes so that the DoF counting is (optionally) done during run time. This is relevant for operators and functions for which the sizes are not known at compile time.

Implemented PETSc CG solver wrapper. Changed PETSc MinRes constructor to obtain both relative and absolute tolerance.

- fix: copying BCs to numerator
- added option to not apply any preconditioner to the velocity
- added option to not reassembly the matrix every solve

Fixed idling issue in dedicated agglomeration for Stokes due to collective function calls on world comm in restriction.

The commit renames PETScSparseMatrixcreateMatrixFromFunction() to
createMatrixFromOperator() and does the same for the method with
the Once suffix.

Resolves issue #119.

- set relative tolerances too low to reach
- set default absolute tolerances in minres solvers to 1e-12
- removed Schur prec. test since it takes too long
- minor epsilon fixes