Add common abstract methods in LinkedList

[czgdp1807]

Description of the problem

Some methods which are common to all the child classes of LinkedList aren't defined as abstract methods in the superclass itself. To fix this issue such methods should be added.
See the following template,

def name_of_abstract_method(self, arg1, arg2, ..., argn):
    raise NotImplementedError("This is an abstract method")

If you will go through other files such methods are pretty common to locate.

Example of the problem

References/Other comments