code: def conc(self,H=70.,Om=0.3,overdens=200.,wrtcrit=False,
               ro=None,vo=None):
          
          if ro is None: ro= self._ro
          if vo is None: vo= self._vo
          try:
              return self.rvir(H=H,Om=Om,overdens=overdens,wrtcrit=wrtcrit,
                               ro=ro,vo=vo,use_physical=False)/self._scale
          except AttributeError:
              raise AttributeError("This potential does not have a '_scale' defined to base the concentration on or does not support calculating the virial radius")
docs: NAME:
  
             conc
  
          PURPOSE:
  
             return the concentration
  
          INPUT:
  
             H= (default: 70) Hubble constant in km/s/Mpc
             
             Om= (default: 0.3) Omega matter
         
             overdens= (200) overdensity which defines the virial radius
  
             wrtcrit= (False) if True, the overdensity is wrt the critical density rather than the mean matter density
             
             ro= distance scale in kpc or as Quantity (default: object-wide, which if not set is 8 kpc))
  
             vo= velocity scale in km/s or as Quantity (default: object-wide, which if not set is 220 km/s))
  
          OUTPUT:
  
             concentration (scale/rvir)
  
          HISTORY:
  
             2014-04-03 - Written - Bovy (IAS)