Why does Elementwise::SolverCG call vector_init(solution) on initial solution

[nfehn]

The solution vector passed to the solve function gets initialized with zero in the elementwise CG solver. It is unclear to me why this is done. See

exadg/include/exadg/solvers_and_preconditioners/solvers/elementwise_krylov_solvers.h

Line 278 in e8462c4

vector_init(solution, M);

When initializing with zero, the subsequent matrix-vector product seems to be superfluous.

I tend to say that the solver should work with the solution vector as passed to the solve function.

@kronbichler what do you think?