Improve test coverage

[ghiggi]

Prework

• I understand and agree to this repository's code of conduct.
• I understand and agree to this repository's contributing guidelines.
• I have already submitted an issue or discussion thread to discuss my idea with the maintainers.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and communicate accordingly:

The PR fulfills these requirements:

• It's submitted to the branch named as follow :
  • Add a reader: reader-<institute>-<campaign>
  • Fix a bug: bugfix-<some_key>-<word>
  • Improve the doc: doc-<some_key>-<word>
  • Add a new feature: feature-<some_key>-<word>
  • Refactor some code: refactor-<some_key>-<word>
  • Optimize some code: optimize-<some_key>-<word>
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• Don't forget to link PR to issue if you are solving one.
• All tests are passing.
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

Related GitHub issues and Pull Requests

• Ref: #

Summary

Please explain the purpose and scope of your contribution.

[codecov]

Codecov Report

Attention: Patch coverage is 96.47059% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.81%. Comparing base (28ae0cd) to head (642ef68).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
disdrodb/l2/empirical_dsd.py	95.00%	2 Missing ⚠️
disdrodb/api/io.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #225      +/-   ##
==========================================
- Coverage   85.90%   85.81%   -0.10%     
==========================================
  Files          90       90              
  Lines        6804     6838      +34     
==========================================
+ Hits         5845     5868      +23     
- Misses        959      970      +11     

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[coveralls]

coverage: 85.666% (-0.08%) from 85.742%
when pulling 591a226 on ghiggi:add-test-units
into 28ae0cd on ltelab:main.

[coveralls]

coverage: 85.652% (-0.09%) from 85.742%
when pulling 642ef68 on ghiggi:add-test-units
into 28ae0cd on ltelab:main.