Documentation, imports, next_generation() arguments, and more

• Updated docstrings to Google format
• Reduced imports to only functions/objects used from packages
• Removed "num_survivors" argument from next_generation()
• Added "log_base" argument to next_generation(), used to scale the reverse log space in parent selection (higher == more skewed towards choosing a selected member towards the front of the list returned by the selection function)
• Changed names of class properties for Members and Population (param_vals to parameters)
• Updated README to reflect changes, add additional documentation on mutation