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 )))

1. 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"