text: Title: Why is a large proportion of candidates in the initial population = None?
  Body: Hello, 
  
  I am working through the example notebook [https://github.com/trevorstephens/gplearn/blob/master/doc/gp_examples.ipynb](url) and am able to reproduce the same results.  
  
  When I inspect the initial population `est_gp._programs[0]` only 139 candidates are a gplearn program and the remainder are `None`. I get similar results of a high proportion of the initial population being set to `None` when applying to my own regression problem. Why is this the case?
  
  Line 222 of [https://github.com/trevorstephens/gplearn/blob/master/gplearn/_program.py](url) (I think this is the right place to look) implies this should never happen. 
  
  Thank you!
  