Fixed usage of statistics=std_dev option in multi-model statistics preprocessors

[schlunma]

Description

Fixes an issue with the multi_model_statistic option statistic=std_dev introduced with #673.

Closes #1475
Related to ESMValGroup/ESMValTool#2521

Link to documentation: https://esmvaltool--1478.org.readthedocs.build/projects/ESMValCore/en/1478/recipe/preprocessor.html#multi-model-statistics

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[sloosvel]

Looks good!

[schlunma]

Thanks Saskia! This also fixes the scipy issue in the doc build

[valeriupredoi]

🅱️ oss level 👍

[valeriupredoi]

do you mind doing the same for Tool pls 🍺