Check days between two person by name

  - People is a hashMap for retrieve fast ( O(1) time cost ).
  - HashMap won't work properly in a concurrent environment.
    If we have this requirement we could use Hastable or CopyOnWriteArrayList.
  - We store the oldest person but we could also sort the list by natural ordering using TreeMap.
    Since is not a requirement to hold the list sorted is not needed.
  - We are not controlling exceptions if the person is not in the hash. We can add this logic after.
  - We will need to create the logic to read from the file, but we will implement the load method after
    the logic is completed.