WHY IS THIS CHANGE NEEDED?:
- when using single_birth_plot routine, user can change the label in
  the legend that indicates birth events

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- remove hard-coding of the 'birth event' label by making it an argument
  with the default of 'birth event'.
- this preserves backwards compatibility as i'm creating a new argument
  and the existing argument names are the same

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- backwards compatibility: existing uses of this routine in scripts will
  produce 'budding event' in plots rather than 'birth event'; if a user
  wants to use 'birth event', they can simply define the argument
  birth_label='birth event'.

EVIDENCE THAT COMMIT WORKS:
- works with a random time series and a binary mask-like numpy array,
  and i can change the legend label

REFERENCES:
- issue #36