LA: Simplify iterative solver implementation namespaces
LA: fix inline doc links to generic types
LA: change builder visibility (now only publicly accessible through Matrix/Vector)
LA: make UnitPreconditioner generic & shared
LA: simplify IPreConditioner, misc iterative solver cleanup
LA: simplify IIterationStopCriterium, drop StopLevel
LA: drop iterative solver IIterator abstraction, simplifications
LA: replace complicated OO iterative solver states with simple enum