Add support for warnings

[cc-a]

Currently the tool is only able to report errors however there are situations where we may want to provide warnings to users instead.

A very basic warning system could be added by:

• Adding a warning attribute (list[str]) to UnitChecker.
• Printing any warnings that have been collected after the checker has run (see unit_static_analyser/__init__.py).
• Add a warning to the code, the only candidate so far is a NotImplementedError in the UnitChecker class.
• Update the test test_overloaded_function_definition_not_implemented to reflect the new behaviour.