relax SVN archive

Added support to the internal PDB structural object for determining connected atoms.

This is for when no CONECT records exist. The method __find_bonded_atom() has been renamed to
__bonded_atom(), and the new method __find_bonded_atoms() has been written. This new method
connects all atoms located a certain distance from each other.

Fixed the number arg unit test of spin.number().

Added 2 more unit vector system tests, this time using the internal PDB reader.

Added some more checks in the unit vector system tests.

Allowed the spin number to be reset to None.

Allowed the number arg in the spin.number() user function to be None.

Added another system test for the calculation of unit XH vectors, but with spin numbers removed.

Fixes for the unit vector system test.

Activated the unit vector system test.

Added a simple system test for the calculation of unit XH bond vectors.