text: Title: saltelli.sample enables user defined skip_values? 
  Body: Hey,
  I was trying to use saltelli.sample and found that the skip_values is not a parameter in the method. The value used is 1000 in the code. It might be good to add skip_values as a parameter.  Or is there any particular reason that skip_values is hidden? 
  Look forward to your reply.
  
  Best wishes,
  Qian