Add parser

Add empty file for parser implementation
Add empty graph.{c,h}
Add node_t data structure and memory operations

Add the node_t srtuct and related memory management operations
to allocate a new node on the heap and delete it.
Add definition of parse_node_descr

Add the definition of parse_node_add, a function used to parse strings
and convert them to node_t structs
Update documentation of parse_node_descr
Start parser implementation
Add header null.h

Portably defines the NULL macro
Merge branch 'master' into parser
Update documentation of parse_node_descr
Add boolean.h
Implement count_number
Add test case for count_numbers
Add another test case for count_numbers
Add function populate_adjacency

Yet to implement
Add a test case for populate_adjacency
Add a test case for parse_node_descr

For the moment it fails
Fix the bug highlighted by unit tests