modified node.py and tree.py : improved documentation

o added a license header to each file
o added at the beginning of each file a list of the classes
o modified some details required by Pylint : e.g. 'return' and 'del' keywords are to be written without parentheses
o Tree and Node classes begin with a list of methods, attributes and class attributes
o almost all lines have a length lesser than 80 characters, except in the license text
o added a very brief documentation to error classes

o many functions have one or two lines of documentation. This documentation is very basic for the moment