Use driverblock option for choosing between matrix-free/matrix-based solver

So far we used the matrix_free form option to decide if we want to use a matrix-free solver. I wanted to build two driverblocks, one matrix-free and one matrix-based for the same forms but this was not possible.

Now the option that decides which solver to use is a driverblock option. Currently there are two options:

• matrix_free: Use a matrix free solver (maybe without preconditioning)
• matrix_free_preconditioner: Use a matrix free preconditioner

I will add a test where we build two driverblocks, one matrix-free and one matrix-based using the same forms.