Add more conversions to GAMS Eps value into GdxWriter

[soininen]

10^-10 and EPS are now converted to Eps.

Resolves #413

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 78.21%. Comparing base (00009ef) to head (4ce8191).

Files	Patch %	Lines
spinedb_api/spine_io/exporters/gdx_writer.py	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #414 +/- ## ========================================== - Coverage 78.23% 78.21% -0.02% ========================================== Files 72 72 Lines 9515 9517 +2 Branches 2031 2032 +1 ========================================== Hits 7444 7444 - Misses 1735 1737 +2 Partials 336 336 ```

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