Return average SASA of the atoms.

def get_total_per_atom_sasa(self):
		"""Return average SASA of the atoms."""
		total_sasa = defaultdict(int)
		for traj in range(len(self.atom_sasa)):
			for atom in self.atom_sasa[traj]:
				total_sasa[atom]+=float(sum((self.atom_sasa[traj][atom])))/len(self.atom_sasa[traj][atom])
		for atom in total_sasa:
			total_sasa[atom]=float(total_sasa[atom])/len(self.atom_sasa)
		return total_sasa