1123 population data for different years

[patricklnz]

Changes and Information

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

-Population Data can now be downloaded for past years, specified with ref_year
-The test for total population has been adjusted to be 5% accurate to 8.4 million

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

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• Every addressed issue is linked (use the "Closes #ISSUE" keyword below)
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closes #1123

[HenrZu]

Thank you really much for the fast integration! Very nice as always :)

Only some short comments. And some tests would be great.

[mknaranja]

The incorrect population data makes our ESID scenario pipeline failing and this should be repaired soon.

@patricklnz Are you still adding some unit tests as Henrik requested? That'd be great! Could you also provide some information on what you did in the issue?
@HenrZu Did you do a full review? Could you otherwise please do soon?

[HenrZu]

Looks good, however some unit tests would still be fine. Also, the param ref_year is only mentioned in the parameters. It would be cool if you could add some more documentation :)