Iatiflattener

Added

- Several unit tests have been added to cover budget and transaction calculations.

Fixed

- Bug in calculation of value_local for some transactions: https://github.com/iati-data-access/iati-flattener/issues/9
- Budget splitting: budgets are now split by calculating a day rate, which is then used
to work out the amount for each budget period. https://github.com/iati-data-access/iati-flattener/issues/16

Fixed

- Date quarters: incorrect start dates of quarters: https://github.com/iati-data-access/iati-flattener/issues/7
- Budget splitting: budgets were not split correctly across quarters when budget periods spanned calendar years:
https://github.com/iati-data-access/iati-flattener/issues/8