Restructure code to improve maintainability

Currently, maintaining or editing the material is a little difficult, as changes in one section carry forward to changes elsewhere. It would help a bit more if the changes in each section were more isolated - so that each section's changes could be implemented as a separate branch, making it easier to do.

The files that are touched are:

1. **Collaborative Development**
- 1.4: `inflammation/models.py`: Minor edit just to demo `git`.
- 1.5: `inflammation-analysis.py`, `inflammation/models.py`: Improving code quality.
- 1.6: `inflammation/models.py`, `inflammation/views.py`: Improving code quality (nonspecific).
2. **Ensuring Correctness**
- 2.1: `tests/test_models.py`: Expanding tests for models.
- 2.4: `tests/test_models.py`, `inflammation/models.py`: Expanding tests, fixing errors in patient normalisation.
3. **Software Development as a Process**
- 3.2. `full-data-analysis` branch: Adds new code for running analysis.
- 3.3: `inflammation/models.py`: Adding new data source.
- 3.3: `inflammation/compute_data.py`, `tests/test_compute_data.py`: Refactoring to class-based load.
- 3.4: `inflammation/compute_data.py`, `test/test_compute_data.py`: Refactor analysis to allow regression testing.
- 3.5: `inflammation/compute_data.py`: Refactoring into MVC model.
4. **Development for Re-Use**
- 4.1: `feature-std-dev` branch: Adds new code for plotting standard deviation.
- 4.1: `inflammation-analysis.py`, `inflammation/models.py`, `test/test_models.py`: Editing the submitted code.
5. **Managing Software**
- Nothing specific.

My proposal would be to replace `inflammation/models.py` with a submodule, `inflammation/models`. Then when we add a new data source, we can do it by only editing one file, e.g. `inflammation/models/csv_data.py`. Likewise, splitting tests into separate files per module. This could be done either by starting with it split up, or splitting it into multiple files as part of section 3.