Filter the gcc's messages to differentiate errors from warnings.

Modern version of gcc (mine is 9.3) emits a lot of warnings & hints in stderr. The compiler backing in CompAut is currently considering that every messages in an error which is a bit too much. In this PR I differentiate the error messages from the compiler (the one that should raise en exception) from the warnings. In order to allow the developpers to see the compiler's code warning an extra attribute is added to TemplateCompiler to get the messages.

Example of use: M.compile(comp_dir="comp_dir", clean=False) # Generates the binaries print(M.compilation_messages) # Displays there is warnings