Add MessageCode to Logger to simplify unit testing

[clinssen]

Right now, messages such as warnings and errors are recorded in the Logger. So we can do e.g.:

(Pdb) Logger.get_all_messages_of_level_and_or_neuron(model.get_neuron_list()[0], LoggingLevel.WARNING)[0]
(<pynestml.meta_model.ast_neuron.ASTNeuron object at 0x7f4232793898>, <LoggingLevel.WARNING: 1>, 'Input block not defined!')
(Pdb) Logger.get_all_messages_of_level_and_or_neuron(model.get_neuron_list()[0], LoggingLevel.WARNING)[1]
(<pynestml.meta_model.ast_neuron.ASTNeuron object at 0x7f4232793898>, <LoggingLevel.WARNING: 1>, 'Output block not defined!')
(Pdb) Logger.get_all_messages_of_level_and_or_neuron(model.get_neuron_list()[0], LoggingLevel.WARNING)[2]
(<pynestml.meta_model.ast_neuron.ASTNeuron object at 0x7f4232793898>, <LoggingLevel.WARNING: 1>, "Variable 'eV' has the same name as a physical unit!")

Unit tests are often checking for the number of warnings (i.e. the length of this list), not what the warnings are saying. See https://github.com/nest/nestml/blob/master/tests/cocos_test.py for an example. This means that when we add a warning for something, we have to touch a lot of unrelated unit tests.

We should additionally record the MessageCode (from https://github.com/nest/nestml/blob/master/pynestml/utils/messages.py#L997) in the Logger tuple, to simplify unit testing. Each test could then check for the presence or absence of a particular MessageCode rather than the length of the warnings (or errors) list.