Add: avl function attributes
Add: nonnull macros for avl/common.h
Merge pull request #251 from matthiasbeyer/add-avl_attributes

Add attributes to avl functions
Add: Implementation for ll_union()
Add: test for ll_union()
Add: Test for ll_union() with overlapping values
Outsourced set type definition in own header file
Added warning about errornous operation
Added implementation for r_set_union()
Add: Interface definition for ht_union()
Add stub for method avl_union()
Add helper function merge_tree() for avl_union()
Implement avl_union
Add: Simple avl_union() test
Add: test for avl_union() with overlapping values
Add prelimary implementation for ht_union()
Check for the existence of a node before inserting a new one