Add tests #5
Description
At least one test should check the I/O works:
https://github.com/hdrake/xbudget/blob/df74dcb8d4927d6f71426d28b7fa4dc51811257d/xbudget/presets.py#L4C37-L5
A more comprehensive test could use the Baltic test data, which we could download as in xwmt:
https://github.com/hdrake/xwmt/blob/0a2c9053b24115963c2c5e022a50f49e173ff961/xwmt/tests/conftest.py#L9-L13