Extensive use of forward class declaration.

Basically I replaced all objects included in other objects with references, so as
to eliminate the need to include the corresponding header file. In this manner the
entanglement of compilation is greatly reduced.
Forward class declaration is now used everywhere excepted for:
* cases where it is impossible due to inline functions
  (for efficiency or because they are templates)
* cases where it is impossible due to inheritance
* small objects of fixed size such as Vector/Tensor/AtomNumber/Units/Pbc
* classes of std:: namespace (it is not possible to forward-declare them).