说明：  修改单链表模板类，使结点类的数据域改为数据类对象的指针。要求为该单链表模板类建立深复制函数，这需要结点类和学生类的复制构造函数均为深复制。其中学生类的深复制是调用string类的复制构造函数完成的，直接用默认的，同样析构函数也不要重编。 并据此建立一个学生链表，原始数据放在文本文件EXP19_2.txt中。
(Modify the linked list template class, so that node data fields to the data class class object. Requirements for the establishment of a single linked list template class deep copy function, which requires node class and the students copy the class are deep copy constructor. Deep copy of which class of students is to call the copy constructor string class completed, the direct use of the default, the same destructor do not restated. And to establish a student list, the original data in the text file EXP19_2.txt.)