APCON Purpose
  
  This function returns the values of many different constants used
     in astrophysics. Category
  
  Astrophysics Calling Sequence
  
   result = APCON(value, [/list, /cgs, /mks])
Inputs
  
  value: A string naming the desired constant. For a list of 
  
  supported constants, use print, APCON(/list)
Keyword Parameters
  
  LIST: If set, return a list of the possible constants 
  
  CGS: If set, return values in cgs units (happens by default) 
  
  MKS: If set, return values in mks units. 
     
Outputs
     The value of the input constant
     Examples
  
   IDL> print, apcon('G', /MKS) 
      
      6.6730000e-11 
      IDL> print, apcon('G')
             6.6730000e-08
Todo
  
  Masses, radii of solar system bodies
Modification History
  
  May 2009: Written by Chris Beaumont