Update FileManager.java

Gabe I added some code to FileManager to have a Serializable style. I think FileManager should have a list of doctors and a list of nurses that will hold any saved vector objects when we load them from the disk. That way we can just manipulate the present vectors and then when were ready we can save them. So far FileManager only has a vector called doctor_list. This will hold the saved vector from listOfDoctors file.