* implement a unified lduLduBase

* change location of lduMatrix based solvers

* Wrap rhs and initial guesses in vectors

* prepare init_initial_guess_vector

* unify solve_impl_ member functions

* prepare independent preconditioner generation

* add basic implementation for reusable preconditioner

* Add separate preconditioner class

* Add IOPreconditioner files

* remove missing folder

* Fix formating, minor fixes

* fix overflow issue for sorting

* Use csr matrix as default matrix format (#25)

* basic test if csr has any impact

* use csr matrix as matrix type