Add exclusion criteria for ENA samples without collection date

[johausmann]

Addresses issue #86

[codecov[bot]]

Codecov Report

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

Comparison is base (b652204) 85.87% compared to head (f362c4e) 85.93%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #144 +/- ## =========================================== + Coverage 85.87% 85.93% +0.05% =========================================== Files 48 48 Lines 5026 5047 +21 =========================================== + Hits 4316 4337 +21 Misses 710 710 ``` | [Files](https://app.codecov.io/gh/TRON-Bioinformatics/covigator/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=TRON-Bioinformatics) | Coverage Δ | | |---|---|---| | [covigator/accessor/abstract\_accessor.py](https://app.codecov.io/gh/TRON-Bioinformatics/covigator/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=TRON-Bioinformatics#diff-Y292aWdhdG9yL2FjY2Vzc29yL2Fic3RyYWN0X2FjY2Vzc29yLnB5) | `100.00% <100.00%> (ø)` | | | [covigator/accessor/ena\_accessor.py](https://app.codecov.io/gh/TRON-Bioinformatics/covigator/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=TRON-Bioinformatics#diff-Y292aWdhdG9yL2FjY2Vzc29yL2VuYV9hY2Nlc3Nvci5weQ==) | `89.37% <100.00%> (+0.70%)` | :arrow_up: | | [covigator/tests/unit\_tests/faked\_objects.py](https://app.codecov.io/gh/TRON-Bioinformatics/covigator/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=TRON-Bioinformatics#diff-Y292aWdhdG9yL3Rlc3RzL3VuaXRfdGVzdHMvZmFrZWRfb2JqZWN0cy5weQ==) | `87.27% <100.00%> (ø)` | | | [covigator/tests/unit\_tests/test\_ena\_accessor.py](https://app.codecov.io/gh/TRON-Bioinformatics/covigator/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=TRON-Bioinformatics#diff-Y292aWdhdG9yL3Rlc3RzL3VuaXRfdGVzdHMvdGVzdF9lbmFfYWNjZXNzb3IucHk=) | `100.00% <100.00%> (ø)` | |

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[johausmann]

This addresses the ENA accessor part related to this issue. I would like to discuss the retroactive removal in our next technical meeting.