extend the function set_local_system_of_units which allows now to use different methods to set the system.

add examples/units/
which gives examples on how to set a system of units in pNbody.

git-svn-id: https://svn.epfl.ch/svn/pnbody@55 f1771ac9-52fb-4361-9f77-cbfb7563c7d4