fixed damping factor for linear systems, fixed buggy return true

there was a mismatch between verbose/non-verbose code for linear systems: in verbose mode the code erroneously used the global damping factor (which is 0.5) while in nonverbose mode it uses no damping (1.0) as it should
fixed a premature return when solving a linear system after solving the first equation, fixed flow of the function and eliminated the related todo
removed redundant, non const functions, added a few const &