Remove unnecessary Sparsity.h include.
Add DenseLinearSystem class.

Uncrustified version of original commit.
Replace valarray with vector in dense LinEqSys.
Replace solution check in TestLinearSystem.

Replace incorrect usage of getSolVec().
Uncrustify TestLinearSystem.cpp.
Use std::size_t in TestLinearSystem.
Add DenseLinearSystem test.
Initialize DenseLinearSystem with 0 upon contruction.
Uncrustify LisLinearSystem.{h,cpp}.
Derive LisLinearSystem from ISystemOfLinearEquations.
Remove setOption() from ISystemOfLinearEquations.

It is not used by DenseLinearSystem.
Remove redundant addSubRHS() for double* sub_vector argument.
Rename private matrix, rhs, and solution for consistency.
Add MatrixType and VectorType template parameters to ISystemOfLinearEquations.
Uncrustify ISystemOfLinearEquations.{h,cpp}.
Move up _dimension and knownSolution vectors.

Also move up common setKnownSolution() methods.
Rename length to common dimension.
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.