Fix and improvement of the QMR solver.

Description

Improvement of the QMR solver.

• For the batchified and non-batchified version the default algorithm can now use a zero starting point. This should be the default but changing this breaks the tests.
• The code now uses BLAS for the non-batchified QMR solver, thus giving for free OpenMP support.
• The batchified code is fully documented, to help following what the code does.

News snippet

Improvement of the QMR solver.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.