Add function definition for parse_graph_file
Add function read_file

Reads a whole file into a buffer in memory.
Add function count_lines

This function counts the lines in a string.
Add function node_init and node_free

These are used to init and free stack-allocated nodes
Add function parse_graph_string
Work on parse_graph_string
Add parse_node_descr_to
Reimplement parse_node_descr

Reimplement parse_node_descr by means of parse_node_descr_to
Implement parse_graph_string
Add test case for parse_graph_string
Correct a bug in count_lines

Correct a bug in count_lines and add more tests.