Make LisLinearSystem::checkError() method const.
Add getMatEntry() to LisLinearSystem.
Add setZero test for LisLinearSystem.
Replace bogous setZero() method in LisLinearSolver.
Add getMatEntry(i,j) method to the base class.
Write printout as operator<< with def. impl. in LinearEquationSystem.
Override printMatrix() for LisLinearSystem.

Printing raw-wise is faster here than the default implementation.
In tests move up LinearEquationSystem<M, V> independent code.

Move random number generators, fill_random(), and check_matrix_sol_rhs_are_zero().