Multigrid solver setup separation

Problem

All multigrid solvers only implement a setupLinearSystem(obj, A, b, x0) method that sets the matrix as well as the right hand side. Conceptually, the matrix can only be set once per level, since the solver is tailored to the problem setting. The right-hand side, however, can, in priciple, change.

Calling setupLinearSystem twice throws an error, because all level-wise setup steps are taken in this method. It would be better to separate the setup of the matrix (as well as all corresponding level-wise setups) and the setup of the RHS.

Suggestion

Split the setupLinearSystem method into two methods:

• setupSystemMatrix that takes care of the matrix, transfer operators, and smoother setup.
• setupRhs that takes care of the right-hand side and an optional initial guess.

Steps

• Modify localAdaptiveMultigrid and test with this file.
• Change the setup logic in whole class hierarchy, starting with IterativeSolver.
• Merge changes into AISFEM-experiments branch & make Zarantonello example run.