Replace _A, _b, and _x with pointers in dense lin. eq. sys.

This allows to move up these later.
Replace _A, _b, and _x with pointers in LIS lin. eq. sys.

This allows to move up these later.
Move up _A, and _b, and _x and corresponding getter methods.
Rename ISystemOfLinearEquations to LinearEquationSystem.

This is a little bit shorter and more direct.
Drop I because it's not an abstract class.