Start work on making models more generic.
Combine Render and Clear classes into single class Output;

These classes were both concerned with the same aspect- sending output to the terminal; with Render originally calling Clear. I've moved these into the same class to allow me to refactor and simplify them later.
Add more documentation.
Include Model in all repository models, also;

- Add #store method to simplify storing of models in repositories.
Move all system events into Events.
Move CursorApp and LinesApp into root of examples directory.
Move #not_found to Repository and remove exceptions;

- Reduce clutter and simplify the codebase by not having named NotFound exceptions.
Continued working on model stuff;

- Use Model #store instead of 'repository.update'.
- Fix some intermittent failing tests.
- Remove 'entity' in favour of 'model'.
- Add test classes and modules to centralize test objects.
Rename local variable for clarity.
Refactor and add some tests.
Move Group into repositories/models.
Make #not_found private.
Refactor Output tests.