Limit observation_period_end_date to current date

[kzollove]

Fixes #39
By limiting the end_date to current date, age groups are created without NAs

PR Checklist

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• Did I make sure to only change the part of the file where I introduced a new change/feature?
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Link to relevant issue(s)
Closes #39

How did you address these issues with this PR? What methods did you use?