Isothermal potential

added some more external potentials to gravity, plus a test for energy and angular momentum conservation in the IsothermalPotential/GravityOnly example.

examples/ExternalPointMass/externalPointMass.yml

@@ -43,4 +43,5 @@ PointMass:
   position_y:      50.
   position_z:      50.	
   mass:            1e10     # mass of external point mass in internal units
+  timestep_mult:   0.03     # controls time step