Add dataset tests

[sphamba]

Prework

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

• Bugfix
• Feature
• Documentation
• Tutorial
• Code style update
• Refactor
• Build-related changes
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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 :
  • Fix a bug: bugfix-<some_key>-<word>
  • Improve the doc: doc-<some_key>-<word>
  • Improve a tutorial tutorial-<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>
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• 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)

Summary

• add script to generate small hdf5 files from downloaded raw files, and netcdf files from open_granule.
  • does not seem to work with all raw files (eg IMERG)
  • relies on start_date to be added in product_def.yml for all products
• add tests for granule
  • test comparing small hdf5 files with saved netcdf file fails
• add tests for groups and variables

[coveralls]

coverage: 47.262% (+5.6%) from 41.632% when pulling 3514c12 on EPFL-ENAC:add_dataset_tests into 2fc2e2b on ghiggi:main.

[ghiggi]

LGTM.