Added an internal `wrap_compound` parameter

I just add this here as a starting point. Anyone can pick up this MR and continue the work.

As discussed in today's DEV meeting this changes the behavior of our unwrapping. The idea is that we introduce a new internal variable wrap_compound that controls how the system is wrapped in unwrapped. For example atomic density modules should use wrap_compound='atoms' while all molecules that require whole molecules should provide the output of get_compound(self._universe.atoms).