Generalise Budget File Saving & Loading
- Extracted generalised interface for loading and storing a budget.
- Extracted generalised interface for errors when loading and storing budgets.
- Extracted generalised interface for loading and storing an exchange rate cache.
- Updated the load and store error handing for a budget file to match the documentation.
- Changed the navigation library to use the budget storage interface instead of the direct class.
- Changed the navigation library to use the exchange rates storage interface instead of the direct class.
- The navigation driver library no longer depends upon the file-storage library.
Closes #22 (closed)
Edited by John Schneiderman