FIX: set fillvalue before applying scale/offset when exporting odim

[kmuehlbauer]

• [x] Closes #122
• [ ] Tests added
• [x] Changes are documented in history.md

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.32%. Comparing base (8937457) to head (f4c1b39).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #173 +/- ## ======================================= Coverage 91.32% 91.32% ======================================= Files 21 21 Lines 4010 4010 ======================================= Hits 3662 3662 Misses 348 348 ``` | [Flag](https://app.codecov.io/gh/openradar/xradar/pull/173/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openradar) | Coverage Δ | | |---|---|---| | [notebooktests](https://app.codecov.io/gh/openradar/xradar/pull/173/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openradar) | `80.57% <0.00%> (ø)` | | | [unittests](https://app.codecov.io/gh/openradar/xradar/pull/173/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openradar) | `89.65% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openradar#carryforward-flags-in-the-pull-request-comment) to find out more.

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[kmuehlbauer]

@mgrover1 If you have any idea how to test this, please let me know. Otherwise this should be good to go.