force storage mode to double at entry point

[Enchufa2]

Fixes #324

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f7088ae) 91.23% compared to head (6f80877) 91.24%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #344 +/- ## ======================================= Coverage 91.23% 91.24% ======================================= Files 19 19 Lines 981 982 +1 ======================================= + Hits 895 896 +1 Misses 86 86 ``` | [Impacted Files](https://codecov.io/gh/r-quantities/units/pull/344?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=r-quantities) | Coverage Δ | | |---|---|---| | [R/conversion.R](https://codecov.io/gh/r-quantities/units/pull/344?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=r-quantities#diff-Ui9jb252ZXJzaW9uLlI=) | `89.28% <100.00%> (+0.12%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=r-quantities). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=r-quantities)

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