Add dir-structure for doubly_linked_list project.

data_structures/doubly_linked_list/(
makefile, src/doubly_linked_list.(c, h))
	-Add doubly_linked_list project directory, and main source
	skeleton files.
Add doubly_linked_list func prototypes/empty defs.

data_structures/doubly_linked_list/src/doubly_linked_list.(c, h)
	-Add prototypes and empty definitions for all of the foreseeably
	necessary `doubly_linked_list` functions.
Impl. createDLList(), freeDLList(), freeDLNode().

data_structures/doubly_linked_list/src/doubly_linked_list.(c, h)
	-Implement and document `createDLList()`, `freeDLList()`, and
	`freeDLNode()`. The first two were renamed from `createSLList()`
	and `freeSLList()`, as they were initially incorrectly declared.
Impl. createDLNode(), insertHead(), testDLList().

data_structures/doubly_linked_list/src/doubly_linked_list.(c, h)
	-Implement and document `createDLNode()` and `insertHead()`.

data_structures/doubly_linked_list/src/test.c
	-Implement and document `testDLList()`, and move all test
	statements from `main()` to it for greater modularity.
	-Redefine `freeData()` as static, because it was unnecessarily
	exposed to the global scope.
Impl. insertHead(), insertTail(), and removeTail().

data_structures/doubly_linked_list/src/doubly_linked_list.(c, h)
	-Implement and document `insertHead()`, `insertTail()`, and
	`removeTail()`.
Impl. insertDLLNodeAtIndex(), removeDLLNodeAtIndex()

data_structures/doubly_linked_list/src/doubly_linked_list.(c, h)
	-Implement and document `insertDLLNodeAtIndex()` and
	`removeDLLNodeAtIndex()`: the doubly-linked list implementation
	is now essentially finished, but must be tested.
Test all doubly_linked_list.c functions.

data_structures/doubly_linked_list/src/doubly_linked_list.(c, h)
	-Move the `DLNode` struct from global to static scope.
	-Redefine `createDLNode()` and `freeDLNode()` as static, and
	move the relevant documentation from `doubly_linked_list.h` to
	`doubly_linked_list.c`.
	-Define `printDLList()`.

data_structures/doubly_linked_list/src/test.c
	-Add statements to `testDLList()` to test various functions
	inside `doubly_linked_list.c`.
Merge branch 'feature/data_structures.doubly_linked_list' into develop

data_structures/doubly_linked_list/
	-Implement a doubly-linked list.