GaussAlgorithm is not inherited from DenseDirectLinearSolver anymore.
Removed class DenseDirectLinearSolver.
Removed class DirectLinearSolver.
Removed class LinearSolver.
Made GaussAlgorithm a template.

The template arguments are matrix type and vector type. For new
matrix types it is necessary to specialize the template. This is
done for DenseMatrix as matrix type and double* as vector type.