New organisation that puts even more of the handling code into the
templates directory

- Files now look like templates/groups/neurongroup/NeuronGroup.*
- .py file contains the CPPMethodHandler class
- .py files are found automatically by importing all the .py files in
the templates directory

This means that we can now add new classes to the standalone code
generation by just modifying the templates/ directory and not touching
the standalone_cpp.py module at all.