Added the macros_directory function that returns the place where the MDANSE macros has to be searched

Modified the Selector class hierarchy with a MMTK.Trajectory instead of a MMTK.Universe when initializing the class

The MDANSE database is now append instead of prended to the MMTK
database search path
Removed unused method

Bug fix in Partial charges plugin

Changed the name of the template script generated with --js option