code: def dens_in_gevcc(vo,ro):
      
      return vo**2./ro**2./_G*_MSOLAR10p30KG*_CIN10p5KMS**2./_EVIN10m19J/ _PCIN10p18CM**3.*10.**-4.
docs: NAME:
  
         dens_in_gevcc
  
      PURPOSE:
  
         convert density to GeV / cm^3
  
      INPUT:
  
         vo - velocity unit in km/s
  
         ro - length unit in kpc
  
      OUTPUT:
  
         conversion from units where vo=1. at ro=1. to GeV/cm^3
  
      HISTORY:
  
         2014-06-16 - Written - Bovy (IAS)