Closes #2231 added check in derive_vars_period() to remove unused columns

[manciniedoardo]

Closes #2231

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[github-actions]

Package	Line Rate	Health
admiral	98%	✔
Summary	98% (4596 / 4684)	✔

[manciniedoardo]

Issue was arising because derive_vars_period() does a lot of dynamic transposing/renaming with dataset_ref and when the latter contains variables not referenced anywhere in the function call, the transposing/renaming fails. Thus I added two lines of code to strip dataset_ref of any variables not relevant for the period derivation right at the start. I also added a test for this eventuality.