Integrated duplicate function to the insert functions of API, and updated client files (main.c, data_type files)

Now the users create one and only dummy object and change it to their needs (integration for the insert functions)
Expanded the API, adding several functions

	Changelog:  Added these functions
		1. dll_size
		2. dll_insert_before (a specified by a key element)
		3. dll_insert_after (a specified by a key element)
		4. dll_delete_front (/head)
		5. dll_delete_back (/tail)
		6. dll_get_front (/head)
		7. dll_get_back (/tail)
		8. dll_insert_front
		9. dll_insert_back
		10. dll_append
	Information for the above, is available at both the DoublyLinkedList_ADT source and header file
	Probably a manual will be added with explicit info about each parameter