Add function get_leastsq_for_reproduction()

This function isolates the process of obtaining the parameters for the
reproduction stage by means of a least squares regression.
Modify function

Use get_leastsq_for_reproduction() in plot_leastsq_for_reproduction()
Change order of plotting
Add function get_reg_for_predator_death()

Use this function in plot_reg_for_predatord_death()
Add function get_reg_for_prey_death()

Use this function in plot_reg_for_prey_death()