LA: iterative solver stop criterium cosmetics, fix potential race (minor)
LA: iterative solver no longer stateful on iterator
LA: iterative solver no longer stateful on preconditioner
LA: s/IPreConditioner/IPreconditioner/ (casing)
LA: drop iterative solver manual-stop functionality (to be replaced)
LA: new iterative solver cancellation stop criterium
LA: simplify iterative solver stop criteria by merging Indeterminate+Running into Continue state
LA: Undo introduction of iterator HasX helpers
LA: integrate iterative solvers into matrix type, leads to significant simplification
LA: leverage lighter params syntax in iterator creation
LA: add direct (factorization-based) Solve methods to matrix type