B+ tree
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   B+ tree is an n-ary tree for effective searching, inserting and deleting values which are
sorted by their keys. A B+ tree consists of a root, internal nodes and leaves.The root may be
either a leaf  or a node with two or more children. A B+ tree is B tree, where each node
contains only keys. Values are stored in leaves. Leaves are in the same depth and they are
interconnected to the linked list.

   The primary value of a B+ tree is in storing data for efficient retrieval. For example
it is used for storing information about IP addresses. Key is an IP address and value
is a structure with information. The key can be any value of specific size, which has to
be specified in advance. Then there has to be function for comparing the keys.

   For using the tree, call the b_plus_tree_initialize() function first. The function
creates all necessary structures for storing and accessing the data. Parameters you give
the function are number of children for node, size of the table, pointer to function for
comparing keys, size of the value, length of the key used for "indexing" stored items

Function for comparing keys has two parameters (two keys which are compared),
returns         zero - a==b,
        negative int - a<b,
        positive int - a>b.

   For inserting or searching there is b_plus_tree_insert_or_find_item() function.
Parameters are pointer to b+ tree structure and key which should be added. The return
value is pointer to memory where the item is stored. If the item is not in the tree,
it is created.

   For removing items there is b_plus_tree_delete_item() function.
Parameters are pointer to b+ tree structure and key which should be deleted. The return
value is 1 when the key was in tree and was successfully deleted, otherwise 0.

   For reading each item, there is function b_plus_tree_create_list_item() with parameter
of tree structure, it returns structure of item (b_plus_tree_item), where the value and key
is stored. For getting next item, call function  b_plus_tree_get_next_item_from_list(), with
parameters pointer to the tree structure and pointer to the item. For removing item from tree there
is b_plus_tree_delete_item_from_list() function with the same parameters
as b_plus_tree_get_next_item_from_list(). For deleting the item structure, call
b_plus_tree_destroy_list_item() with pointer to structure.

   For destruction of the whole tree there is b_plus_tree_destroy() function, parameter is pointer
to the b_plus_tree structure.