Adding a Selection Method

1. Define a selection method function in propeller.selection that selects an individual from the population
2. Add the selection method in propeller.selection/select-parent under the case call:

 (defn select-parent
 "Selects a parent from the population using the specified method."
 [pop argmap]
 (case (:parent-selection argmap)
 :new-selection-method (new-selection-method )))
 ```
3. When runnning a problem, specify the selection method in `:parent-selection`. 
For example:
 ```
 lein run -m propeller.problems.simple-regression :parent-selection :new-selection-method"
 ```