Analysis specific element classes

The direct child classes of the Element class are assumed to be base classes for particular analyses. For example, the StructuralElement class is the base class for all ``structural'' elements. It declares all necessary services required by structural analysis (for example methods computing stiffness matrices, load, strain and stress vectors etc.). This class may provide general implementations of some of these services if possible, implemented by virtual functions (they reflect specific element formulation - like computing element shape functions), which are declared, but their implementation is left on derived classes, which implement specific elements.