Fix: Additional indent = 2
Fix: run astyle on avl/common.c
Added testing file for AVL implementation
Added allocation test for avl test
Added test for testing allocation and deleting for AVL
Added test for adding data to the avl tree
Added test for adding and destroying afterwards
Added test for adding multiple elements
Added test for adding multiple and destroying AVL afterwards
Added test for colliding inserts
Added test for finding a single element
Added test for finding multiple elements
[ci skip] Fixup: test_avl_add: The ptrs to the l/r next element should be NULL, actually
Fixed compiler errors about a const modified
Fixes a bug in the avl_find test

I also updated the documentation of avl.h as
@neithernut forgot to update it.
Changed tests to conform to master
Merge pull request #205 from matthiasbeyer/fix-conditional_indent_astyle

The conditional indent setting for astyle was wrong
Destroy the linked list before removing the node
This fixes the double free on certain glibc vers.
Fix: on a _find() operation, the compare data element should be const