Small change in SchurComplement class.

- SchurComplement is child of LinSys_Petsc
- optimization of SchurComplement constructor
Optimization of SchurComplement inner structure.

- move data of computed schur complement to inner structures
- remove class member Compl
Add new methods of SchurComplement.
Remove old parts of code in SchurComplement class.
Small changes in SchurComplemet class.

- move code for create inversion matrix to separate method
Add methods to SchurComplement.

- constructor without LinSys Orig parameter.
- make_complement_distribution method.
Change set_complement method.
Unit test of SchurComplement.
Remove unused variables of SchurComplement.
Add copy constructors of LinSys and its descendants.
Add complement object of SchurComplement class.
Small change in Schur complement unit test.
Modify of SchurComplement test.
Apply new functionality of Schur complement in DarcyFlow.

- using new constructor
- modification of LinSys structure in DarcyFlow
- optimization of make_schur methods (merging into one)
- modification of update_solution method
Remove Orig class member of Schur complement.
Add assert to copy constructor of LinSys.

- copy constructor of LinSys and its descendants can't be used now
Update solution_precision method.

- get precision of solving in LinSys class and descendants
Optimization of Schur complement.

- add SchurComplement::solve() method
- separation of allocation and computing of linear system
- change of DarcyFlow::update_solution() method
Add flags that indicate change of matrix and right side.
Change method for set complement.

- move parts of code to form_schur method
- optimization of Schur complements creating