add size mediated parent selection for lexicase survival

[lacava]

include option to mediate lexicase survival via size of programs for each selection event.

[lacava]

• add flag for lex_size = True/False
  sizes = [len(i.stack) for i in individuals]
• add sizes arg to ep_lex() in epsilon_lexicase.h
• add size flag to ep_lex() in epsilon_lexicase.h
• change line 103 to take only individuals under the size randomly selected from sizes
• add new arguments to few_lib.pyx