921 Improve PopulationData Download

[patricklnz]

Changes and Information

Please briefly list the changes (main added features, changed items, or corrected bugs) made:

• use requests with authentification and deep link from regionalstatistik.de
• unittest read_data was deleted, since this is not supported for gPd.py

If need be, add additional information and what the reviewer should look out for in particular:

Merge Request - Guideline Checklist

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Checks by code author

• Every addressed issue is linked (use the "Closes #ISSUE" keyword below)
• New code adheres to coding guidelines
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• Tests are added for new functionality and a local test run was successful
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Checks by code reviewer(s)

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Closes #921

[reneSchm]

Nice simplification!